you could try using a custom button, with a small default image & a larger image.
then using the button outlet you could:
Code:
UIImage *btnPressedImg = [UIImage imageNamed:@"largerImage.png"];
[btnOutlet setImage:(UIImage *)btnPressedImg forState:UIControlEventTouchDown];
Im not sure about the fluidity of the animation change...
I'd like to know if anyone has a better suggestion.