点击按钮后弹出图片库选择图片，然后返回给Button的image。
-(IBAction)selectIcon {
UIImagePickerController*imagePicker =[[UIImagePickerController alloc]init];
//设置图库源
[imagePicker setSourceType:UIImagePickerControllerSourceTypePhotoLibrary];
//是否允许编辑图片
[imagePicker setAllowsEditing:YES];
//设置当前控制器为代理
[imagePicker setDelegate:self];
[selfPResentViewController:imagePicker animated:YES completion:nil];
}
选取图片后需实现代理方法didFinishPickingMediaWithInfo
-(void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info{
[_icon setImage:info[UIImagePickerControllerEditedImage] forState:UIControlStateNormal];
[self dismissViewControllerAnimated:YES completion:nil];
}