1、可以指定文字字体、大小和颜色（注:指定的文字在WEB服务器上需要有该字库）；
2、可以加文字阴影；
3、可以指定文字的透明度；
4、可以指定背景图片或背景颜色；
5、可以指定生成的图片大小（宽度与高度）；
6、可以指定文字的位置（左边距和上边距）；
7、当用户设定的文字字号太大，能自动调整文字大小使之能适应生成图片的大小。

using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
namespace Ycweb.Controls.Utility
{
     /**/
     /// <summary>
     /// WaterMark
     /// </summary>
     public class Watermark
     {
          private int _width;
          private int _height;
          private string _fontFamily;
          private int _fontSize;
          private bool _adaptable;
          private FontStyle _fontStyle;
          private bool _shadow;
          private string _backgroundImage;
          private Color _bgColor;
          private int _left;
          private string _resultImage;
          private string _text;
          private int _top;
          private int _alpha;
          private int _red;
          private int _green;
          private int _blue;
          private long _quality;
          public Watermark()
          {
                //
                // TODO: Add constructor logic here
                //
                _width = 460;
                _height = 30;
                _fontFamily = "华文行楷";
                _fontSize = 20;
                _fontStyle = FontStyle.Regular;
                _adaptable = true;
                _shadow = false;
                _left = 0;
                _top = 0;
                _alpha = 255;
                _red = 0;
                _green = 0;
                _blue = 0;
                _backgroundImage = "";
                _quality = 100;
                _bgColor = Color.FromArgb(255, 229, 229, 229);
          }
          /**/
          /// <summary>
          /// 字体
          /// </summary>
          public string FontFamily
          {
                set { this._fontFamily = value; }
          }
          /**/
          /// <summary>
          /// 文字大小
          /// </summary>
          public int FontSize
          {
                set { this._fontSize = value; }
          }
          /**/
          /// <summary>
          /// 文字风格
          /// </summary>
          public FontStyle FontStyle
          {
                get { return _fontStyle; }
                set { _fontStyle = value; }
          }
          /**/
          /// <summary>
          /// 透明度0-255,255表示不透明
          /// </summary>
          public int Alpha
          {
                get { return _alpha; }
                set { _alpha = value; }
          }
          /**/
          /// <summary>
          /// 水印文字是否使用阴影
          /// </summary>
          public bool Shadow
          {
                get { return _shadow; }
                set { _shadow = value; }
          }
          public int Red
          {
                get { return _red; }
                set { _red = value; }
          }
          public int Green
          {
                get { return _green; }
                set { _green = value; }
          }
          public int Blue
          {
                get { return _blue; }
                set { _blue = value; }
          }
          /**/
          /// <summary>
          /// 底图
          /// </summary>
          public string BackgroundImage
          {
                set { this._backgroundImage = value; }
          }
          /**/
          /// <summary>
          /// 水印文字的左边距
          /// </summary>
          public int Left
          {
                set { this._left = value; }
          }
          /**/
          /// <summary>
          /// 水印文字的顶边距
          /// </summary>
          public int Top
          {
                set { this._top = value; }
          }
          /**/
          /// <summary>
          /// 生成后的图片
          /// </summary>
          public string ResultImage
          {
                set { this._resultImage = value; }
          }
          /**/
          /// <summary>
          /// 水印文本
          /// </summary>
          public string Text
          {
                set { this._text = value; }
          }
          /**/
          /// <summary>
          /// 生成图片的宽度
          /// </summary>
          public int Width
          {
                get { return _width; }
                set { _width = value; }
          }
          /**/
          /// <summary>
          /// 生成图片的高度
          /// </summary>
          public int Height
          {
                get { return _height; }
                set { _height = value; }
          }
          /**/
          /// <summary>
          /// 若文字太大，是否根据背景图来调整文字大小，默认为适应
          /// </summary>
          public bool Adaptable
          {
                get { return _adaptable; }
                set { _adaptable = value; }
          }
          public Color BgColor
          {
                get { return _bgColor; }
                set { _bgColor = value; }
          }
          /**/
          /// <summary>
          /// 输出图片质量，质量范围0-100,类型为long
          /// </summary>
          public long Quality
          {
                get { return _quality; }
                set { _quality = value; }
          }
          /**/
          /// <summary>
          /// 立即生成水印效果图
          /// </summary>
          /// <returns>生成成功返回true,否则返回false</returns>
          public bool Create()
          {
                try
                {
                     Bitmap bitmap;
                     Graphics g;
                     //使用纯背景色
                     if (this._backgroundImage.Trim() == "")
                     {
                          bitmap = new Bitmap(this._width, this._height, PixelFormat.Format64bppArgb);
                          g = Graphics.FromImage(bitmap);
                          g.Clear(this._bgColor);
                     }
                     else
                     {
                          bitmap = new Bitmap(Image.FromFile(this._backgroundImage));
                          g = Graphics.FromImage(bitmap);
                     }
                     g.SmoothingMode = SmoothingMode.HighQuality;
                     g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                     g.CompositingQuality = CompositingQuality.HighQuality;
                     Font f = new Font(_fontFamily, _fontSize, _fontStyle);
                     SizeF size = g.MeasureString(_text, f);
                     // 调整文字大小直到能适应图片尺寸
                     while (_adaptable == true && size.Width > bitmap.Width)
                     {
                          _fontSize--;
                          f = new Font(_fontFamily, _fontSize, _fontStyle);
                          size = g.MeasureString(_text, f);
                     }
                     Brush b = new SolidBrush(Color.FromArgb(_alpha, _red, _green, _blue));
                     StringFormat StrFormat = new StringFormat();
                     StrFormat.Alignment = StringAlignment.Near;
                     if (this._shadow)
                     {
                          Brush b2 = new SolidBrush(Color.FromArgb(90, 0, 0, 0));
                          g.DrawString(_text, f, b2, _left + 2, _top + 1);
                     }
                     g.DrawString(_text, f, b, new PointF(_left, _top), StrFormat);
                     bitmap.Save(this._resultImage, ImageFormat.Jpeg);
                     bitmap.Dispose();
                     g.Dispose();
                     return true;
                }
                catch
                {
                     return false;
                }
          }
     }
}