给UIButton添加背景图片
//定义一个button
UIButton * button = [UIButton buttonWithType:(UIButtonTypeRoundedRect)];
//大小
button.frame =CGRectMake(10, 10, 50, 50);
//设置成圆形（根据需要）
button.layer.cornerRadius = button.frame.size.width /2;
button.clipsToBounds = YES;
//调用添加图片的方法
[button setBackgroundImage:[UIImage imageNamed:@"add.png"] forState:(UIControlStateNormal)];
[self.view addSubview:button];
