Make sure the method you're using to transparent will work. The one way I always did it was using
GiFFY.
Make sure the image you create is saved in the 24-bit BMP format, open it with GiFFY, transparent, and save as a GIF. The image has to be somewhere in the Graal directory, and this is where it would go wrong for me. The image would have to be directly downloaded from the server (entering a level with that image in it) for it to appear in graalshop. I believe it has something to do with the filenamecache.txt file.