I am not sure if my answer will solve your problem or not.  I have had the odd problem with transparency myself.  I have found the usual culprit is anti aliasing.  Because antialiasing tries to make your edges look smootherte by blending with the surounding colour, what often hapens when you set the background to transparent, is you end up with an outline similar to what you seem to have.

The ways around this really differ from graphics package to package.  My only tip is I always use an outlandish colour as a background when I am dealing with transparencies, and if I am drawing/pasting/writing, I always make sure anti aliasing is off.

Hope this helps

Mark