/** <summary>
    /// 生成缩略图
    /// </summary>
    /// <param name="originalImagePath">源图路径（物理路径）</param>
    /// <param name="thumbnailPath">缩略图路径（物理路径）</param>
    /// <param name="width">缩略图宽度</param>
    /// <param name="height">缩略图高度</param>
    /// <param name="mode">生成缩略图的方式</param>   
    public static void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string mode)
    {
        System.Drawing.Image originalImage = System.Drawing.Image.FromFile(originalImagePath);

int towidth = width;
        int toheight = height;

int x = 0;
        int y = 0;
        int ow = originalImage.Width;
        int oh = originalImage.Height;

switch (mode)
        {
            case "HW"://指定高宽缩放（可能变形）               
                break;
            case "W"://指定宽，高按比例                   
                toheight = originalImage.Height * width / originalImage.Width;
                break;
            case "H"://指定高，宽按比例
                towidth = originalImage.Width * height / originalImage.Height;
                break;
            case "Cut"://指定高宽裁减（不变形）               
                if ((double)originalImage.Width / (double)originalImage.Height > (double)towidth / (double)toheight)
                {
                    oh = originalImage.Height;
                    ow = originalImage.Height * towidth / toheight;
                    y = 0;
                    x = (originalImage.Width - ow) / 2;
                }
                else
                {
                    ow = originalImage.Width;
                    oh = originalImage.Width * height / towidth;
                    x = 0;
                    y = (originalImage.Height - oh) / 2;
                }
                break;
            default:
                break;
        }

//新建一个bmp图片
        System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);

//新建一个画板
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);

//设置高质量插值法
        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

//设置高质量,低速度呈现平滑程度
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

//清空画布并以透明背景色填充
        g.Clear(System.Drawing.Color.Transparent);

//在指定位置并且按指定大小绘制原图片的指定部分
        g.DrawImage(originalImage, new System.Drawing.Rectangle(0, 0, towidth, toheight),
            new System.Drawing.Rectangle(x, y, ow, oh),
            System.Drawing.GraphicsUnit.Pixel);

try
        {
            //以jpg格式保存缩略图
            bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
        }
        catch (System.Exception e)
        {
            throw e;
        }
        finally
        {
            originalImage.Dispose();
            bitmap.Dispose();
            g.Dispose();
        }
    }

/** <summary>
    /// 在图片上增加文字水印
    /// </summary>
    /// <param name="Path">原服务器图片路径</param>
    /// <param name="Path_sy">生成的带文字水印的图片路径</param>
    protected void AddShuiYinWord(string Path, string Path_sy)
    {
        string addText = "测试水印";
        System.Drawing.Image image = System.Drawing.Image.FromFile(Path);
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(image);
        g.DrawImage(image, 0, 0, image.Width, image.Height);
        System.Drawing.Font f = new System.Drawing.Font("Verdana", 16);
        System.Drawing.Brush b = new System.Drawing.SolidBrush(System.Drawing.Color.Blue);

g.DrawString(addText, f, b, 15, 15);
        g.Dispose();

image.Save(Path_sy);
        image.Dispose();
    }

/** <summary>
    /// 在图片上生成图片水印
    /// </summary>
    /// <param name="Path">原服务器图片路径</param>
    /// <param name="Path_syp">生成的带图片水印的图片路径</param>
    /// <param name="Path_sypf">水印图片路径</param>
    protected void AddShuiYinPic(string Path, string Path_syp, string Path_sypf)
    {
        System.Drawing.Image image = System.Drawing.Image.FromFile(Path);
        System.Drawing.Image copyImage = System.Drawing.Image.FromFile(Path_sypf);
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(image);
        g.DrawImage(copyImage, new System.Drawing.Rectangle(image.Width - copyImage.Width, image.Height - copyImage.Height, copyImage.Width, copyImage.Height), 0, 0, copyImage.Width,copyImage.Height, System.Drawing.GraphicsUnit.Pixel);
        g.Dispose();

image.Save(Path_syp);
        image.Dispose();
    }
}

转自:http://www.cnblogs.com/xiongeee/archive/2007/03/30/693568.html