Cerebral Vortex Software Development is happy to give you a 68000 routine for unpacking PNG pictures.
That's not a C portage but a full routine written from nothing with for only support, the only one webpage on Png format.
It can read, 1,2,4,8,16 bits by pixel, interlaced or not, Alpha or not. That's surely not the fastest one, but the smallest one.
Less than 5 kilobytes, and can be surely a core for doing a fast 060 version.

Hope that will be helpful for all 68000 users.

Everything here : http://cerebral-vortex.net/index.php?id=94

(Questions can be asked on Dbug forum in coding section too)

Azrael, GT Turbo and Zerosquare (C.V.S.D.)