問題大概就像這樣

（請無視那紅字）
一般而言這是高亮狀態的設定有所缺乏。完善程式碼如下：

[_pupilBtn setImage:[UIImage imageNamed:@"a1"]
             forState:UIControlStateNormal];
[_pupilBtn setImage:[UIImage imageNamed:@"a1"]
             forState:UIControlStateHighlighted];
[_pupilBtn setImage:[UIImage imageNamed:@"a2"]
             for
 
State:UIControlStateSelected];
[_pupilBtn setImage:[UIImage imageNamed:@"a2"]
             forState:UIControlStateSelected | UIControlStateHighlighted];