I had the same problem. I see this question is a little old, but I want to provide a clear and correct answer to save other folks (like me) some time when it pops up in their search results.
It took me a bit of searching and experimenting, but I found the solution. Simply set the ContentMode of the “hidden” ImageView that is inside the UIButton.
[[firstButton imageView] setContentMode: UIViewContentModeScaleAspectFit];
[firstButton setImage:[UIImage imageNamed:imageName] forState:UIControlStateNormal];
Perhaps that’s what Dan Ray was alluding to in his accepted answer, but I suspect not.