currentBackgroundImage：獲取按鈕背景圖片

NSData *imagedata1=UIImagePNGRepresentation(btn.currentBackgroundImage);//按鈕背景圖片轉NSData
    NSData *imagedata2=UIImagePNGRepresentation([UIImage imageNamed:@"資訊預設"]);//未選圖片轉NSData
    if([imagedata1 isEqualToData:imagedata2]){
      //
    }