我们知道，如果网站上传图片时，如果用户上传的是CMYK图片，那么在网站上将是无法显示的，通常的现象是出现一个红叉。
下面使用将Image重新绘制为Format24bppRgb的方式来解决此问题：

public static void SavePostedImage(HttpPostedFile postedFile, string destFileName, int maxHeight, int maxWidth)
{
     System.Drawing.Imaging.ImageFormat imgFormat;
     if (destFileName.ToLower().EndWith("jpg"))
     {
          imgFormat = ImageFormat.Jpeg;
     }
     else //这里可以加更多选项，比如png,gif,tif....
     {
          imgFormat = ImageFormat.Gif;
     }

Bitmap bmp = new Bitmap(postedFile.InputStream);

if (IsCMYK(bmp))
     {
          bmp = ConvertCMYK(bmp);
     }

if ((bmp.HorizontalResolution > 72) || (bmp.VerticalResolution > 72))
     {
          bmp.SetResolution(72, 72);
     }

Bitmap saveBmp;
     if ((bmp.Height > maxHeight) || (bmp.Width > maxWidth))
     {
          Double heightRatio = Convert.ToDouble(maxHeight) / Convert.ToDouble(bmp.Height);
          Double widthRatio = Convert.ToDouble(maxWidth) / Convert.ToDouble(bmp.Width);
          Double scaleRatio;

if (heightRatio > widthRatio)
          {
               scaleRatio = widthRatio;
          }
          else
          {
               scaleRatio = heightRatio;
          }

int height = Convert.ToInt32(bmp.Height * scaleRatio);
          int width = Convert.ToInt32(bmp.Width * scaleRatio);

saveBmp = new Bitmap(bmp, width, height);
     }
     else
     {
          saveBmp = new Bitmap(bmp);
     }

bmp.Dispose();
     saveBmp.Save(destFileName, imgFormat);
     saveBmp.Dispose();
     postedFile.InputStream.Close();
}

public static string GetImageFlags(System.Drawing.Image img)
{
     ImageFlags FlagVals = (ImageFlags)Enum.Parse(typeof(ImageFlags), img.Flags.ToString());
     return FlagVals.ToString();
}

public static bool IsCMYK(System.Drawing.Image img)
{
     bool isCmyk;

if ((GetImageFlags(img).IndexOf("Ycck") > -1) || (GetImageFlags(img).IndexOf("Cmyk") > -1))
     { isCmyk = true; }
     else
     { isCmyk = false; }

return isCmyk;
}

public static Bitmap ConvertCMYK(Bitmap bmp)
{
     Bitmap tmpBmp = new Bitmap(bmp.Width, bmp.Height, PixelFormat.Format24bppRgb);

Graphics g = Graphics.FromImage(tmpBmp);
     g.CompositingQuality = CompositingQuality.HighQuality;
     g.SmoothingMode = SmoothingMode.HighQuality;
     g.InterpolationMode = InterpolationMode.HighQualityBicubic;

Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
     // 将CMYK图片重绘一遍,此时GDI+自动将CMYK格式转换为RGB了
     g.DrawImage(bmp, rect);

Bitmap returnBmp = new Bitmap(tmpBmp);

g.Dispose();
     tmpBmp.Dispose();
     bmp.Dispose();

return returnBmp;
}

更多讨论:
(1)如何将RGB图片转换为CMYK图片?
(2)如何将RGB图片转换为索引图片?
(3)如何将RGB图片转换为灰度/黑白线条图片?
(4)上述转换如果是在WPF中,又如何进行呢?

留给读者去思考吧.