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.

>>19
Then you just use raw uncompressed images. Storage space and RAM capacity are no longer an issue. RLE is slower because you have to convert to a RAW format to display it anyway.

For distribution over the internet, using bz2 or lzma for your entire distributable is as good as PNG compression.