This is rather a funny problem I have ever come across,
I have a table layout with 9 image buttons, 3 per row. Every ImageButton has a different image associated with it. I have set the background of the image button to transparent (#00000000). Now here is where the funny stuff happens, One of the images doesn't show up on the emulator (Gingerbread) as well as a device running Froyo. The layout editor shows all the images in place.
Here are some more stuff:
Do you still have the problem. I had a similar bug - a particular drawable was not being displayed, no matter in what
Is it the case that your image is the first (alphabetically) in the drawable folder? For some strange reason there is a problem with the first drawable, i "solved" my problem by adding a dummy drawable in first place. Still I'm very curious where the actual problem is.
Hope that helps! And I'm looking forward for any further explanations :)