Best. Method. Ever.

gistfile1.m

-(void)changeButtonBackgroundWithAnimation:(UIButton *)button andBackground:(UIImage *)background withDuration:(CFTimeInterval)aduration {
    [button setImage:background forState:UIControlStateNormal];
    
    CATransition *transition = [CATransition animation];
    transition.duration = aduration;
    transition.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut];
    transition.type = kCATransitionFade;
    
    [button.layer addAnimation:transition forKey:nil];
}