Hey demian.Not to be naive, but have you tried this?
Haven't tried this, but try making the PNG partially transparent if you haven't already. I'll check the exporter settings to see if there's anything about it.EDIT: Make sure "Translucent" is selected for the material. That should do it.
Make sure you check off ztrans,bring the alpha down to whatever you want,check off transluctive and additive.On GIMP make sure you check off everything on the exporting as a .png.
Translucent addictive.