IOS7 关于UIButton setImage setBackgroundImage无效的问题

高于IOS7的场合

UIButton *btn = [[UIButton alloc] initWithFrame:CGRectMake(5, 100,30, 30)];
[btn setImage:[UIImage imageNamed:@"logo"] forState:UIControlStateNormal];

IOS7的场合

UIButton *btn = [[UIButton alloc] initWithFrame:CGRectMake(5, 100,30, 30)];
[btn setImage:[UIImage imageNamed:@"logo.png"] forState:UIControlStateNormal];