I am using a .tga image for the system buttons in a skin I'm working on, imported from a .png with alpha-trans. Only the four corner pixels are transparent. The image attributes are set correctly as best I can tell - transparency & translucency are both enabled. However, on the Logoff & Shutdown dialogs the corner pixels are not transparent against the screen mask background:

Anyone know how to get those corner pixels to be translucent? The button paints correctly everywhere else; only the End Session dialogs have this problem.
Thanks.