private void MakeSLT(string oldImagePath,string newImagePath)
{
//oldImagePath -原图地址 newImagePath 生成缩略图地址
   int width = 150;//缩略图的宽度
   int height = 112;// 缩略图的高度
   int level = 100; //缩略图的质量 1-100的范围

System.Drawing.Image oldimage = System.Drawing.Image.FromFile(oldImagePath);
System.Drawing.Image thumbnailImage = oldimage.GetThumbnailImage(width, height,new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback), IntPtr.Zero);
Bitmap bm=new Bitmap(thumbnailImage);
//处理JPG质量的函数
   ImageCodecInfo[] codecs=ImageCodecInfo.GetImageEncoders();
ImageCodecInfo ici=null;
foreach(ImageCodecInfo codec in codecs)
{
if(codec.MimeType=="image/jpeg")
ici=codec;
}
EncoderParameters ep=new EncoderParameters();
ep.Param[0]=new EncoderParameter(Encoder.Quality,(long)level);
bm.Save(newImagePath,ici,ep);
}

  VB.NET版本：

Sub makeSLT(ByVal oldImagePath As String,ByVal newImagePath As String)
Dim oimg As System.Drawing.Image = System.Drawing.Image.FromFile(oldImagePath)
Dim nimg As System.Drawing.Image = oimg.GetThumbnailImage(wids, heis, Nothing, IntPtr.Zero)
Response.Clear()
Dim outs As Bitmap = New Bitmap(nimg)
'处理图像质量
        Dim codecs As ImageCodecInfo() = ImageCodecInfo.GetImageEncoders()
Dim ici As ImageCodecInfo ' = System.DBNull
        For Each codec As ImageCodecInfo In codecs
If codec.MimeType = "image/jpeg" Then
ici = codec
End If
Next
Dim ep As EncoderParameters = New EncoderParameters
ep.Param(0) = New EncoderParameter(System.Drawing.Imaging.Encoder.Quality, CLng(100))
outs.Save(newImagePath, ici, ep)
End Sub

  第二段：根据上传图片重新生成新图片
C#：

public string UploadImage(System.Web.HttpPostedFile postFile)
{
try
{
if(postFile.ContentLength==0)
{
throw(new Exception("NotExistedPostFile"));
}
else
if(postFile.ContentLength>Convert.ToInt32(System.Configuration.ConfigurationSettings.AppSettings["MaxImageSize"]))
{
throw(new Exception("TooLargePostFile"));
}
String strExt=System.IO.Path.GetExtension(postFile.FileName);
if(System.Configuration.ConfigurationSettings.AppSettings["ImageExtension"].IndexOf(";"+strExt+";")>-1)
{
throw(new Exception("ErrorImageExtension"));
}
String strFileName=@"/uploads/images/"+DateTime.Now.ToString("yyyyMMddHHmmssffff")+".jpg";
String strAbsoluteFileName=System.Web.HttpContext.Current.Server.MapPath(strFileName);
String Copyright=System.Configuration.ConfigurationSettings.AppSettings["CopyRight"];
System.Drawing.Image imgPhoto=System.Drawing.Image.FromStream(postFile.InputStream,true);
//取高和宽
    int phWidth = imgPhoto.Width;
int phHeight =imgPhoto.Height;
//建新图,指定格式为每像素 24 位；红色、绿色和蓝色分量各使用 8 位。
    Bitmap bmPhoto = new Bitmap(phWidth, phHeight,PixelFormat.Format24bppRgb);
//设置分辨率
    bmPhoto.SetResolution(imgPhoto.HorizontalResolution,imgPhoto.VerticalResolution);
//准备Graphics
    Graphics grPhoto = Graphics.FromImage(bmPhoto);
try
{
//指定消除锯齿的呈现。
     grPhoto.SmoothingMode = SmoothingMode.AntiAlias;
//拷贝原图到做图区
     grPhoto.DrawImage(
imgPhoto,
new Rectangle(0, 0, phWidth, phHeight),
0,
0,
phWidth,
phHeight,
GraphicsUnit.Pixel);
//用指定Sizes绘制图片时，测量用指定的字串宽度
//取可能的最大宽度
     int[] sizes = new int[]{64,48,32,16,8,6,4};
Font crFont = null;
SizeF crSize = new SizeF();
for (int i=0 ;i<7; i++)
{
crFont = new Font("Verdana", sizes[i],
FontStyle.Bold);
crSize = grPhoto.MeasureString(Copyright,
crFont);
if((ushort)crSize.Width < (ushort)phWidth)
break;
}
//指定做图点
     int yPixlesFromBottom = (int)(phHeight *.05);
float yPosFromBottom = ((phHeight -
yPixlesFromBottom)-(crSize.Height/2));
float xCenterOfImg = (phWidth/2);
StringFormat StrFormat = new StringFormat();
StrFormat.Alignment = StringAlignment.Center;
//绘制copyright
     SolidBrush semiTransBrush2 =
new SolidBrush(Color.FromArgb(100, 0, 0,0));
grPhoto.DrawString(Copyright,
crFont,
semiTransBrush2,
new PointF(xCenterOfImg+1,yPosFromBottom+1),
StrFormat);
SolidBrush semiTransBrush = new SolidBrush(
Color.FromArgb(100, 255, 255, 255));
grPhoto.DrawString(Copyright,
crFont,
semiTransBrush,
new PointF(xCenterOfImg,yPosFromBottom),
StrFormat);
bmPhoto.Save(strAbsoluteFileName,ImageFormat.Jpeg);
}
finally
{
grPhoto.Dispose();
bmPhoto.Dispose();
imgPhoto.Dispose();
}
return strFileName;
}
catch(Exception excep)
{
throw(excep);
}
}