I have a method that does it for me.
The method takes UIButton
and makes the image aspect fit.
-(void)makeImageAspectFitForButton:(UIButton*)button{
button.imageView.contentMode=UIViewContentModeScaleAspectFit;
button.contentHorizontalAlignment=UIControlContentHorizontalAlignmentFill;
button.contentVerticalAlignment=UIControlContentVerticalAlignmentFill;
}