what about http://pngdelphi.sourceforge.net/ ?

It even works without Delphi VCL, which would be a great portability point and make it useful for games. the only problem is, that it uses delphi's zlib OBJ files, which efficiently bind it to delphi-only, but making it use PasZlib shouldn't be a problem imo - just replace PNGZLIB.PAS with another unit, that will use paszlib.