Nick Post mentioned in a comment that he's had problems trying to get images in the 2D panels that have an alpha channel to rotate. An astute observation! I had to try this out myself with an XML gauge to see why it doesn't work, and sure enough, this code path simply isn't implemented. In a debug build of flight sim (which I run most of the time) I get an assert which alerts me to this fact. Note that you can successfully shift an image element that has alpha; the code paths diverge significantly for shifts vs. rotations. Unfortunately, I don't see any way to work around this issue in FS9, but a bug has been logged for future reference.
I haven't tried using an image with alpha in a C-style gauge yet, but based on the code I see, it looks it shouldn't work either.
Thanks for pointing this out, Nick.