i got the same problem, wojtek. and i resolve it in photoshop by converting the thumbnail with the "save for web" option instead of "save as" and then jpg, like i did before. this makes a big difference in file size i never noticed before.... i can save a thumbnail now with 30% - 60% quality and the file is still small enough for a "renderosity-compatible" thumbnail :-). it seems like the "save for web" option generates a better optimized file.