Custom background images are not showing on iOS

I have a simple test project that creates some buttons and skins them with a background image. The buttons show fine with their skins in the Game view inside the Unity IDE, but when I build and run on an iPad none of the background images show (although fonts, colours etc are respected). I have made sure the textures are POT, imported as GUI textures, compressed as PVRTC, but none of these make any difference.

I'm sure I'm missing something obvious (Unity n00b that I am) but am currently baffled and looking for a clue!