第一步：设置按钮背景图片，并且图片随按钮大小变化

第二步：设置按钮使之只显示图片

button1.FlatStyle = FlatStyle.Flat;//style
   button1.ForeColor = Color.Transparent;//前景
   button1.BackColor = Color.Transparent;//去背景
 button1.FlatAppearance.BorderSize = 0;//去边线
 //button1.FlatAppearance.MouseOverBackColor = Color.Transparent;//鼠标经过
//button1.FlatAppearance.MouseDownBackColor = Color.Transparent;//鼠标按下