[Admins]

I have checked the problem, the image seems to be handled ok.
I guess the problem is that npcs have effect mode 0 by default (light effect), so if the npc has an alpha value below 1 or is using a png with alpha layer then by default it is displayed like a light effect. You need to do seteffectmode(1) to get a normal alpha effect.
Changing the default behaviour is not simple since most scripts that display light effects just change the colors (alpha) and don't set the effect mode. It's a little bit annoying for pngs with alpha layer though