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.

Now that I think about it, an easy way to test LZW as used in GIF is to compare the resulting GIF with a raw uncompressed image, and with a PNG that does not use filters (of course all of them using indexed 8-bit color).

Benchmarking image (de)compressors is hairy though because they're too fast, so I'm not going to try.

But anyway:
RAW:         2074368
GIF (LZW):    106162
LZO:           92569
PNG (DEFLATE): 74035

LZO is the RAW image compressed with LZOP -9. Add a handful of bytes for headers if you want to be fair. (The palette was included)