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.

>>82
DEFLATE compresses better but is slower than LZW. There's nothing wrong with that.
It's wrong, if you want realtime speed. Compression rate isnt proportional to compression speed.