ios开发之UIButton 点击两张图标转换

现在想定义一个UIButton, 这个button有两张图片, 一张算是背景图, 另外一张显示图片1.
         我通过setBackgroundImage设置了背景图片, 用setImage 来设置显示图片1. 但是因为图片1大于了我设置的按钮的大小, 所以显示出来的效果是图片1超出了按钮的框架.....
         不知道该怎么弄了?! 想看看大家是怎么弄的?!

          情况就像下面的图片一样:
         
          第一个按钮是两张图片重叠的时候, 后面两个是分别单独加载的时候...毕业论文 
          源代码是这样的:

01.UIButton *btn1 = [UIButton buttonWithType:UIButtonTypeCustom];

02.        [btn1 setFrame: CGRectMake(10, 10, 50, 50)];

03.        [btn1 setBackgroundImage:[UIImage imageNamed: @"ui4.png"] forState:UIControlStateNormal];

04.        [btn1 setImage:[UIImage imageNamed:@"icon0.png"] forState:UIControlStateNormal];

05.        btn1.contentMode = UIViewContentModeScaleAspectFill;

06.        [self addSubview: btn1];

07.       

08.        UIButton *btn2 = [UIButton buttonWithType:UIButtonTypeRoundedRect];

09.        [btn2 setFrame: CGRectMake(65, 10, 50, 50)];

10.        [btn2 setBackgroundImage:[UIImage imageNamed: @"ui4.png"] forState:UIControlStateNormal];

11.        btn2.contentMode = UIViewContentModeScaleAspectFill;

12.        [self addSubview: btn2];

13.       

14.        UIButton *btn3 = [UIButton buttonWithType:UIButtonTypeRoundedRect];

15.        [btn3 setFrame: CGRectMake(120, 10, 50, 50)];

16.        btn2.contentMode = UIViewContentModeCenter;

17.        [btn3 setBackgroundImage:[UIImage imageNamed:@"icon0.png"] forState:UIControlStateNormal];

18.        [self addSubview: btn3];