Winform Button控件添加背景图片, 去掉边框

            button1.FlatStyle = FlatStyle.Flat;
            //button1.FlatAppearance.BorderColor = Color.Transparent;// 不能设置成透明
            button1.FlatAppearance.BorderSize = 0;
            button1.FlatAppearance.MouseDownBackColor = Color.Transparent;