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.

>>7
Nope. You decompress part of sprite only when you need it, then cache the result. Like Diablo 2 does: it decompresses only single direction (which is delta-compressed).