On iOS 6 SDK I wrote the following lines of code to display an image inside a button:
NSURL *thumbURL2 = [NSURL URLWithString:@\"http://example.com/thumbs/2.
There's a good chance that the image is there and you just can't see it. Try changing the button's type to UIButtonTypeCustom. If that doesn't work, set the button's background color to [UIColor clearColor];
UIButtonTypeCustom
[UIColor clearColor];