Custom certificate (plugin)

I've been playing around with this (great) module.

One thing I can't quite figure out is how to create a high-quality signature and save as png8 that will allow transparency (so the certificate background shows through).

I can get transparency, but the limitation of png8 seems to results in a black/transparent image with no grey (so the signature appears 'blocky').

I'm using GIMP and saving as .png (Indexed rather than RGB).
Perhaps it's a GIMP format issue?

Anyone else managed this?

Thanks, Stuart.