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.

>>8
It's not retarded, it's actually smart. If you intend to use 3D hardware for 2D sprite rendering, packing sprites into texture atlases is the only way to keep your draw calls and state switching down and performance at optimum. You might not think performance matters for 2D games, but better performance means less time spent per frame, and if you synch to vertical refresh rate it means less overall power consumption, which is good for games on mobile devices.