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.

>>35
You can always convert 8-bit paletted images to 32-bit RGBA during preprocessing when packing your data for your final build,
Why should I?

on modern hardware,
What if I have no hardware support on my PC (Linux, Reactos or Haiku) or dont want to bloat my software with DirectX?