It's best to combine images that share colors so the sprite can stay under 255 colors. Might have to do this in CoolRunnings.

Another thing that should reduce file-size is having the option to generate PNG-8 files instead of PNG-24. - Maybe you can make it by default when all images doesn't have 8-bit transparency. - You could even combine images that have 8-bit transparency and the ones that doesn't.

PS: If you drag images from different color ranges to the same sprite it should change the output file to PNG-24 if the number of colors exceed 256 and maybe alert the user that this is going to happen.

This tool is definitely useful and really cool. Thanks for sharing and Keep the good work!

re:comment 1:

From the PNG perspective, the transparency palette chunk can have as few entries as there are translucent colors--anywhere from 1 to 255--so it should be possible to combine transparent palette images with non-transparent images with really minor size increase.

Even in IE 6, adding transparency at one end of a palette shouldn't impact the display of opaque images in the same sprite.