I don't think png iconmodeule uses libpng.so it's much older than the shared objects system...I make a new fresh OS4,1 install in a new partition.
After that, the scaling icons works fine. The PNG icons shows like a PNG images.
Copy PNGiconmodule and the correct png1.2 so library.
Nothin in Enhancer could screw up icon scaling.PNG icons now shows well and scaling works. So the problem is in a later installation. I think it goes for the Enhanced Pack, I will install it in the new partition and comment you.
Try comparing the versions of LIBS:workbench.library and LIBS:icon.library betwen the two installs.
Also try copy the ENVARC:Sys/workbench.prefs from the working partition to the broken one see if that fixes it.