Is there any good library for lossless image encoding/decoding that has compression rate more or less similar to PNG but decoding to raw RGB bitmap data would be much faster than PNG?

Also alpha transparency is needed, but not essential because, alpha channel could be taken from separate image.

Original problem lies in slowness of reading and decoding PNG files on iPhone using standard libraries. Obvious and the simples solution would have been storing raw RGB bitmap data, but then size of unpacked ipa is too large - 4 times larger than PNG files. So, I am trying to find some compromise solution.