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.
This worked for me
[myButton1 setBackgroundImage:[UIImage imageNamed:@"phones.png"] forState:UIControlStateNormal];
Note:Remove front image before doing this.