实现代码：

/// <summary>
/// 给图片添加水印代码
/// by boxuming
/// </summary>
/// <returns></returns>
public Image AddWatermark(Image img)
{
	//创建Graphic对象，核心的类
	var myGraphic = Graphics.FromImage(img); 
	//水印内容
	var sourceString = $"http://www.boxuming.com";
	//水印字体
	var font = new Font("Arial", 14);
	//水印的尺寸
	var size = myGraphic.MeasureString(sourceString, font);
	//设置水印的属性：内容、字体、颜色、位置
	myGraphic.DrawString(sourceString, font, System.Drawing.Brushes.WhiteSmoke,new PointF(img.Width - size.Width - 5, img.Height - size.Height - 5));
	//释放myGraphic
	myGraphic.Dispose();
	
	return img;
}