http://bugs.winehq.org/show_bug.cgi?id=25549
--- Comment #16 from Vincent Povirk madewokherd@gmail.com 2011-09-20 11:42:50 CDT --- Gdiplus only uses DIBs for images without alpha, so the content of the alpha channel shouldn't matter. Also, gdiplus only blits to HDCs or HWNDs it gets from the program, never to DIBs that it creates.
So that suggests to me that in this case gdiplus doesn't know anything more about the alpha channel than gdi32 does.