diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8495afa..4789a3c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -64,12 +64,12 @@ find_package(PkgConfig REQUIRED)
 
 if (EXTERNAL_LIB_PATH)
 	set(CMAKE_PREFIX_PATH ${EXTERNAL_LIB_PATH})
-	
+
 	if(EXISTS "${EXTERNAL_LIB_PATH}/${LIB_PATH}/pkgconfig/")
 		SET(ENV{PKG_CONFIG_PATH} "${EXTERNAL_LIB_PATH}/${LIB_PATH}/pkgconfig/")
 	endif()
 	if(APPLE)
-		set(PLATFORM_SHARED_LIBS 
+		set(PLATFORM_SHARED_LIBS
 			libSDL2.dylib libSDL2_image-2.0.0.dylib libSDL2_ttf-2.0.0.dylib libSDL_sound-1.0.1.dylib
 			libfreetype.6.dylib libsigc-2.0.0.dylib
 			CACHE STRING "List of shared libraries that need to be copied into the OS X bundle")
@@ -404,7 +404,7 @@ target_include_directories(${PROJECT_NAME} PRIVATE
 	deps/libsigc++
 	deps/pixman/pixman
 	deps/physfs/src
-	deps/mruby/include	
+	deps/mruby/include
 
 	${SDL2_INCLUDE_DIRS}
 	${FLUID_INCLUDE_DIRS}
@@ -435,11 +435,11 @@ SET(ASYNCIFY "-s ASYNCIFY=1 -s 'ASYNCIFY_IMPORTS=[\"load_file_async_js\"]'")
 
 SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${EMS_FLAGS} ${ERR_FLAGS} ${ASYNCIFY}")
 SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${EMS_FLAGS} ${ERR_FLAGS} ${ASYNCIFY}")
-SET(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${EMS_FLAGS} ${ASYNCIFY} -lopenal -s DISABLE_EXCEPTION_CATCHING=1 -s ASSERTIONS=0 -s SAFE_HEAP=0 -s MINIFY_HTML=0  --shell-file extra/shell.html -s EMULATE_FUNCTION_POINTER_CASTS=0 -s ALLOW_MEMORY_GROWTH=1 -s MAX_WEBGL_VERSION=2")
+SET(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${EMS_FLAGS} ${ASYNCIFY} -lopenal -s DISABLE_EXCEPTION_CATCHING=1 -s ASSERTIONS=0 -s SAFE_HEAP=0 -s MINIFY_HTML=0  --shell-file extra/shell.html -s EMULATE_FUNCTION_POINTER_CASTS=0 -s ALLOW_MEMORY_GROWTH=1 -s MAX_WEBGL_VERSION=2 -s EXPORTED_FUNCTIONS='[\"_main\",\"_reloadBitmap\"]' -s EXPORTED_RUNTIME_METHODS='[\"ccall\",\"cwrap\"]'")
 
 set_target_properties(
     ${PROJECT_NAME}
-    PROPERTIES 
+    PROPERTIES
         OUTPUT_NAME "mkxp"
         SUFFIX ".html"
 )
diff --git a/extra/js/drive.js b/extra/js/drive.js
index ed8748f..78b0e76 100644
--- a/extra/js/drive.js
+++ b/extra/js/drive.js
@@ -19,7 +19,13 @@ function _base64ToBytes(base64) {
     return bytes;
 }
 
-window.loadFileAsync = function(fullPath, callback) {
+// Canvas used for image generation
+var generationCanvas = document.createElement('canvas')
+
+window.loadFileAsync = function(fullPath, bitmap, callback) {
+    // noop
+    callback = callback || (() => {});
+
     // Make cache object
     if (!window.fileAsyncCache) window.fileAsyncCache = {};
 
@@ -27,7 +33,7 @@ window.loadFileAsync = function(fullPath, callback) {
     if (window.fileAsyncCache.hasOwnProperty(fullPath)) return callback();
 
     // Show spinner
-    if (window.setBusy) window.setBusy();
+    if (!bitmap && window.setBusy) window.setBusy();
 
     // Get mapping key
     const mappingKey = fullPath.toLowerCase().replace(new RegExp("\\.[^/.]+$"), "");
@@ -46,18 +52,47 @@ window.loadFileAsync = function(fullPath, callback) {
     const path = "/game/" + mappingValue.substring(0, mappingValue.lastIndexOf("/"));
     const filename = mappingValue.substring(mappingValue.lastIndexOf("/") + 1).split("?")[0];
 
-    // Delete original file if existent
-    try {
-        FS.unlink(path + "/" + filename);
-    } catch (err) {}
+    // Main loading function
+    const load = (cb1) => {
+        getLazyAsset(iurl, filename, (data) => {
+            // Delete original file if existent
+            try { FS.unlink(path + "/" + filename); } catch (err) {}
 
-    // Get the new file
-    getLazyAsset(iurl, filename, () => {
-        FS.createPreloadedFile(path, filename, iurl, true, true, function() {
-            window.fileAsyncCache[fullPath] = 1;
-            if (window.setNotBusy) window.setNotBusy();
-            if (window.fileLoadedAsync) window.fileLoadedAsync(fullPath);
-            callback();
+            FS.createPreloadedFile(path, filename, new Uint8Array(data), true, true, function() {
+                window.fileAsyncCache[fullPath] = 1;
+                if (!bitmap && window.setNotBusy) window.setNotBusy();
+                if (window.fileLoadedAsync) window.fileLoadedAsync(fullPath);
+                callback();
+                if (cb1) cb1();
+            }, console.error);
+        });
+    }
+
+    // Show progress if doing it synchronously only
+    if (bitmap && bitmapSizeMapping[mappingKey]) {
+        // Remove existing file
+        try { FS.unlink(path + "/" + filename); } catch (err) {}
+
+        // Get image
+        const sm = bitmapSizeMapping[mappingKey];
+        generationCanvas.width = sm[0];
+        generationCanvas.height = sm[1];
+
+        // Create dummy from data uri
+        FS.createPreloadedFile(path, filename, generationCanvas.toDataURL(), true, true, function() {
+            // Return control to C++
+            callback(); callback = () => {};
+
+            // Lazy load and refresh
+            load(() => {
+                const reloadBitmap = Module.cwrap('reloadBitmap', 'number', ['number'])
+                reloadBitmap(bitmap);
+            });
         }, console.error);
-    });
+    } else {
+        if (bitmap) {
+            console.warn('No sizemap for image', mappingKey);
+        }
+        load();
+    }
 }
diff --git a/extra/make_mapping.sh b/extra/make_mapping.sh
index 8965c86..5ed3acc 100755
--- a/extra/make_mapping.sh
+++ b/extra/make_mapping.sh
@@ -1,6 +1,7 @@
 #!/bin/bash
 
 echo "var mapping = {" > mapping.js
+echo "var bitmapSizeMapping = {" > bitmap-map.js
 
 for file in {*,*/*,*/**/*}
 do
@@ -17,7 +18,16 @@ fi
 
 echo "\"$fl\": \"${file}?h=${md5}\"," >> mapping.js
 
+if [ -f $file ]
+then
+    sz=`identify -format "%w,%h" "${file}" 2>/dev/null`
+    if [ $? -eq 0 ]; then
+        echo "\"$fl\": [${sz}]," >> bitmap-map.js
+    fi
+fi
+
 done
 
 echo "};" >> mapping.js
+echo "};" >> bitmap-map.js
 
diff --git a/extra/shell.html b/extra/shell.html
index ff790a8..2a0c2d3 100644
--- a/extra/shell.html
+++ b/extra/shell.html
@@ -8,6 +8,7 @@
     <script src="js/localforage.min.js"></script>
     <script src="js/drive.js"></script>
     <script src="gameasync/mapping.js"></script>
+    <script src="gameasync/bitmap-map.js"></script>
 
     <title>MKXP</title>
     <style>
@@ -292,6 +293,7 @@
       var hideTimer = 0;
       function getLazyAsset(url, filename, callback) {
           const xhr = new XMLHttpRequest();
+          xhr.responseType = "arraybuffer";
           const pdiv = document.getElementById("progress");
           let showTimer = 0;
           let abortTimer = 0;
@@ -308,10 +310,9 @@
                       pdiv.style.opacity = '0';
                       hideTimer = 0;
                   }, 500);
-                  callback();
-
                   clearTimeout(showTimer);
                   clearTimeout(abortTimer);
+                  callback(xhr.response);
               }
           }
           xhr.onprogress = function (event) {
diff --git a/src/bitmap.cpp b/src/bitmap.cpp
index 4ecbc0b..51f570f 100644
--- a/src/bitmap.cpp
+++ b/src/bitmap.cpp
@@ -137,7 +137,7 @@ struct BitmapPrivate
 	}
 
 	void addTaintedArea(const IntRect &rect)
-	{	
+	{
 		IntRect norm = normalizedRect(rect);
 		pixman_region_union_rect
 		        (&tainted, &tainted, norm.x, norm.y, norm.w, norm.h);
@@ -258,8 +258,22 @@ struct BitmapOpenHandler : FileSystem::OpenHandler
 
 Bitmap::Bitmap(const char *filename)
 {
+	strcpy(this->filename, filename);
+	this->loadFromFilename();
+}
+
+void Bitmap::loadFromFilename()
+{
+	char * filename = this->filename;
+
 #ifdef __EMSCRIPTEN__
-	load_file_async_js(filename);
+	bool reloading = false;
+	if (this->p) {
+		this->releaseResources();
+		reloading = true;
+	} else {
+		load_file_async_js(filename, (int)this);
+	}
 #endif
 
 	BitmapOpenHandler handler;
@@ -267,7 +281,7 @@ Bitmap::Bitmap(const char *filename)
 	SDL_Surface *imgSurf = handler.surf;
 
 	if (!imgSurf) {
-		printf("ERROR OCCURED LOADING IMAGE %s : %s\n", filename, SDL_GetError());
+		printf("Error occured loading image %s : %s\n", filename, SDL_GetError());
 		throw Exception(Exception::SDLError, "Error loading image '%s': %s",
 		                filename, SDL_GetError());
 	}
@@ -306,6 +320,11 @@ Bitmap::Bitmap(const char *filename)
 	}
 
 	p->addTaintedArea(rect());
+
+#ifdef __EMSCRIPTEN__
+	if (reloading && this->reloadCallback)
+		this->reloadCallback(this->reloadCallbackData);
+#endif
 }
 
 Bitmap::Bitmap(int width, int height)
@@ -1051,7 +1070,7 @@ void Bitmap::drawText(const IntRect &rect, const char *str, int align)
 			outline = TTF_RenderUTF8_Blended(font, str, co);
 
 		p->ensureFormat(outline, SDL_PIXELFORMAT_ABGR8888);
-		SDL_Rect outRect = {OUTLINE_SIZE, OUTLINE_SIZE, txtSurf->w, txtSurf->h}; 
+		SDL_Rect outRect = {OUTLINE_SIZE, OUTLINE_SIZE, txtSurf->w, txtSurf->h};
 
 		SDL_SetSurfaceBlendMode(txtSurf, SDL_BLENDMODE_BLEND);
 		SDL_BlitSurface(txtSurf, NULL, outline, &outRect);
@@ -1334,3 +1353,15 @@ void Bitmap::releaseResources()
 
 	delete p;
 }
+
+#ifdef __EMSCRIPTEN__
+extern "C" {
+int reloadBitmap(int intptr) {
+	Bitmap * bitmap = (Bitmap*) intptr;
+	if (bitmap->isDisposed()) return 0;
+
+	bitmap->loadFromFilename();
+	return 1;
+}
+}
+#endif
diff --git a/src/bitmap.h b/src/bitmap.h
index 732e9c6..3cf0cb4 100644
--- a/src/bitmap.h
+++ b/src/bitmap.h
@@ -119,11 +119,16 @@ public:
 
 	sigc::signal<void> modified;
 
+	char filename[512];
+	void loadFromFilename();
+	void (*reloadCallback)(void *) = NULL;
+	void * reloadCallbackData = NULL;
+
 private:
 	void releaseResources();
 	const char *klassName() const { return "bitmap"; }
 
-	BitmapPrivate *p;
+	BitmapPrivate *p = 0;
 };
 
 #endif // BITMAP_H
diff --git a/src/emscripten.cpp b/src/emscripten.cpp
index 9fa1317..af98cc8 100644
--- a/src/emscripten.cpp
+++ b/src/emscripten.cpp
@@ -2,9 +2,9 @@
 
 #ifdef __EMSCRIPTEN__
 
-EM_JS(void, load_file_async_js, (const char* fullPathC), {
+EM_JS(void, load_file_async_js, (const char* fullPathC, int bitmap), {
 	Asyncify.handleSleep(function(wakeUp) {
-    	window.loadFileAsync(UTF8ToString(fullPathC), wakeUp);
+    	window.loadFileAsync(UTF8ToString(fullPathC), bitmap, wakeUp);
 	});
 });
 
diff --git a/src/emscripten.hpp b/src/emscripten.hpp
index 3ba6dda..57c38a1 100644
--- a/src/emscripten.hpp
+++ b/src/emscripten.hpp
@@ -5,7 +5,7 @@
 #include <emscripten.h>
 
 extern "C" {
-	void load_file_async_js(const char* fullPathC);
+	void load_file_async_js(const char* fullPathC, int bitmap=0);
 
 	void save_file_async_js(const char* fullPathC);
 }
diff --git a/src/tilemap.cpp b/src/tilemap.cpp
index 8b61ea7..c607dea 100644
--- a/src/tilemap.cpp
+++ b/src/tilemap.cpp
@@ -1147,6 +1147,14 @@ DEF_ATTR_RD_SIMPLE(Tilemap, Visible, bool, p->visible)
 DEF_ATTR_RD_SIMPLE(Tilemap, OX, int, p->origin.x)
 DEF_ATTR_RD_SIMPLE(Tilemap, OY, int, p->origin.y)
 
+void invalidateBitmap(void * tilemap) {
+	((Tilemap *) tilemap)->invalidate();
+}
+
+void Tilemap::invalidate() {
+	p->invalidateAtlasContents();
+}
+
 void Tilemap::setTileset(Bitmap *value)
 {
 	guardDisposed();
@@ -1159,6 +1167,9 @@ void Tilemap::setTileset(Bitmap *value)
 	if (!value)
 		return;
 
+	value->reloadCallback = invalidateBitmap;
+	value->reloadCallbackData = (void *) this;
+
 	p->invalidateAtlasSize();
 	p->tilesetCon.disconnect();
 	p->tilesetCon = value->modified.connect
diff --git a/src/tilemap.h b/src/tilemap.h
index 9afe00f..eccf653 100644
--- a/src/tilemap.h
+++ b/src/tilemap.h
@@ -54,6 +54,7 @@ public:
 	~Tilemap();
 
 	void update();
+	void invalidate();
 
 	Autotiles &getAutotiles();
 	Viewport *getViewport() const;