Sadly no & I thought it was just a skin specific issue

Thanks for the screenshot.
Will have to play around with libpng this week to see what's different between 1.5.x (as used by 5.666) and 1.6.x (as used by WACUP and what I'd prefer to keep using due to read optimisations and some general tweaks that make it easier to work with imho).
-dro
[edit]I've just quickly tried dropping in libpng 1.5.28 (latest 1.5.x release) instead of 1.6.29 and the issue is still there. So either
a) that's not representative of what's needed or as is more likely the case now I've done this quick check
b) I'm doing something wrong with the output from the library.