There's a newer version of optipng out (.47) which is faster than the current in portage and can recover from some uncommon image errors. Reproducible: Always Steps to Reproduce:
I see this is ~x86 now. I'd like to report it works for me. In fact, it works better than 0.4.5--I have some 8-bit grayscale PNGs that optipng-0.4.5 destroyed for me, but not for the original author. (USE="-debug -ext-png -ext-zlib +mmx" in emerge -pv output.) Please consider marking this stable ASAP.
0.4.8 in portage, closing a stale bug.