Are there any open source sprite format?

PNG just plainly sucks. I has no support for animations, timing or delta compression and sequence names. Also, PNG is slow at random access (you have to decompress image before use). Proprietary formats (like Diablo 2 DCC) are much better, but highly specialized for their games: some have no true color, other have no directions.

I think, I will use LZW compressed SEXPs for my sprite format. It will allow bignums for width and height, unlimited extensibility and instant Lisp support. Of course, I'll have to do something other than plain ASCII.