diff --git a/src/app/document_api.cpp b/src/app/document_api.cpp
index 3374d27ef..56b0508c0 100644
--- a/src/app/document_api.cpp
+++ b/src/app/document_api.cpp
@@ -173,15 +173,30 @@ void DocumentApi::setPixelFormat(Sprite* sprite, PixelFormat newFormat, Ditherin
   // Use the rgbmap for the specified sprite
   const RgbMap* rgbmap = sprite->getRgbMap(frame);
 
+  // Get the list of cels from the background layer (if it
+  // exists). This list will be used to check if each image belong to
+  // the background layer.
+  CelList bgCels;
+  if (sprite->getBackgroundLayer() != NULL)
+    sprite->getBackgroundLayer()->getCels(bgCels);
+
   for (c=0; c<sprite->getStock()->size(); c++) {
     old_image = sprite->getStock()->getImage(c);
     if (!old_image)
       continue;
 
+    bool is_image_from_background = false;
+    for (CelList::iterator it=bgCels.begin(), end=bgCels.end(); it != end; ++it) {
+      if ((*it)->getImage() == c) {
+        is_image_from_background = true;
+        break;
+      }
+    }
+
     new_image = quantization::convert_pixel_format
       (old_image, newFormat, dithering_method, rgbmap,
        sprite->getPalette(frame),
-       sprite->getBackgroundLayer() != NULL);
+       is_image_from_background);
 
     replaceStockImage(sprite, c, new_image);
   }
diff --git a/src/raster/quantization.cpp b/src/raster/quantization.cpp
index 9d29a34e0..592d3b91f 100644
--- a/src/raster/quantization.cpp
+++ b/src/raster/quantization.cpp
@@ -89,7 +89,7 @@ Image* convert_pixel_format(const Image* image,
                             DitheringMethod ditheringMethod,
                             const RgbMap* rgbmap,
                             const Palette* palette,
-                            bool has_background_layer)
+                            bool is_background_layer)
 {
   // RGB -> Indexed with ordered dithering
   if (image->getPixelFormat() == IMAGE_RGB &&
@@ -223,7 +223,7 @@ Image* convert_pixel_format(const Image* image,
             ASSERT(dst_it != dst_end);
             c = *src_it;
 
-            if (c == 0 && !has_background_layer)
+            if (!is_background_layer && c == image->getMaskColor())
               *dst_it = 0;
             else
               *dst_it = rgba(rgba_getr(palette->getEntry(c)),
@@ -243,7 +243,7 @@ Image* convert_pixel_format(const Image* image,
             ASSERT(dst_it != dst_end);
             c = *src_it;
 
-            if (c == 0 && !has_background_layer)
+            if (!is_background_layer && c == image->getMaskColor())
               *dst_it = 0;
             else {
               r = rgba_getr(palette->getEntry(c));
@@ -268,7 +268,7 @@ Image* convert_pixel_format(const Image* image,
             ASSERT(dst_it != dst_end);
             c = *src_it;
 
-            if (c == image->getMaskColor())
+            if (!is_background_layer && c == image->getMaskColor())
               *dst_it = dstMaskColor;
             else {
               r = rgba_getr(palette->getEntry(c));
diff --git a/src/raster/quantization.h b/src/raster/quantization.h
index f3aae6440..cb28e7bce 100644
--- a/src/raster/quantization.h
+++ b/src/raster/quantization.h
@@ -44,7 +44,7 @@ namespace raster {
                                 DitheringMethod ditheringMethod,
                                 const RgbMap* rgbmap,
                                 const Palette* palette,
-                                bool has_background_layer);
+                                bool is_background_layer);
 
   } // namespace quantization
 } // namespace raster
diff --git a/src/raster/sprite.cpp b/src/raster/sprite.cpp
index 5fe5f8193..bbd9e6149 100644
--- a/src/raster/sprite.cpp
+++ b/src/raster/sprite.cpp
@@ -135,15 +135,20 @@ bool Sprite::needAlpha() const
 void Sprite::setTransparentColor(color_t color)
 {
   m_transparentColor = color;
+
+  for (int i=0; i<m_stock->size(); i++) {
+    Image* image = m_stock->getImage(i);
+    if (image != NULL)
+      image->setMaskColor(color);
+  }
 }
 
 int Sprite::getMemSize() const
 {
-  Image *image;
-  int i, size = 0;
+  int size = 0;
 
-  for (i=0; i<m_stock->size(); i++) {
-    image = m_stock->getImage(i);
+  for (int i=0; i<m_stock->size(); i++) {
+    Image* image = m_stock->getImage(i);
     if (image != NULL)
       size += image->getRowStrideSize() * image->getHeight();
   }