转自csdn论坛，正确已经测试，非常好用。只不过引用的时候有点问题，图片没有释放，见下面红字部分。

我抄一份代码给你,清清月儿的.  

using

System;

using

System.IO;

using

System.Drawing;

using

System.Drawing.Imaging;

public

class

ImageThumbnail

{

   

public

Image ResourceImage;

   

private

int

ImageWidth;

   

private

int

ImageHeight;

   

public

string

ErrorMessage;

   

public

ImageThumbnail(

string

ImageFileName)

    {

        ResourceImage

=

Image.FromFile(ImageFileName);

        ErrorMessage

=

""

;

    }

   

public

bool

ThumbnailCallback()

    {

       

return

false

;

    }

   

//

方法1，按大小

   

public

bool

ReducedImage(

int

Width,

int

Height,

string

targetFilePath)

    {

       

try

        {

            Image ReducedImage;

            Image.GetThumbnailImageAbort callb

=

new

Image.GetThumbnailImageAbort(ThumbnailCallback);

            ReducedImage

=

ResourceImage.GetThumbnailImage(Width, Height, callb, IntPtr.Zero);

            ReducedImage.Save(@targetFilePath, ImageFormat.Jpeg);

            ReducedImage.Dispose();

           

return

true

;

        }

       

catch

(Exception e)

        {

            ErrorMessage

=

e.Message;

           

return

false

;

        }

    }

   

//

方法2，按百分比  缩小60% Percent为0.6 targetFilePath为目标路径

   

public

bool

ReducedImage(

double

Percent,

string

targetFilePath)

    {

       

try

        {

            Image ReducedImage;

            Image.GetThumbnailImageAbort callb

=

new

Image.GetThumbnailImageAbort(ThumbnailCallback);

            ImageWidth

=

Convert.ToInt32(ResourceImage.Width

*

Percent);

            ImageHeight

=

(ResourceImage.Height)

*

ImageWidth

/

ResourceImage.Width;

//

等比例缩放

            ReducedImage

=

ResourceImage.GetThumbnailImage(ImageWidth, ImageHeight, callb, IntPtr.Zero);

            ReducedImage.Save(@targetFilePath, ImageFormat.Jpeg);

            ReducedImage.Dispose();

           

return

true

;

        }

       

catch

(Exception e)

        {

            ErrorMessage

=

e.Message;

           

return

false

;

        }

    }

}

后台代码：

using

System;

using

System.Data;

using

System.Configuration;

using

System.Collections;

using

System.Web;

using

System.Web.Security;

using

System.Web.UI;

using

System.Web.UI.WebControls;

using

System.Web.UI.WebControls.WebParts;

using

System.Web.UI.HtmlControls;

public

partial

class

_Default : System.Web.UI.Page

{

   

protected

void

Page_Load(

object

sender, EventArgs e)

    {

    }

   

protected

void

bt_upload_Click(

object

sender, EventArgs e)

    {

       

try

        {

           

if

(FileUpload1.PostedFile.FileName

==

""

)

            {

               

this

.lb_info.Text

=

"

请选择文件！

"

;

            }

           

else

            {

               

string

filepath

=

FileUpload1.PostedFile.FileName;

               

string

filename

=

filepath.Substring(filepath.LastIndexOf(

"

\\

"

)

+

1

);

               

string

serverpath1

=

Server.MapPath(

"

images/

"

)

+

filename;

               

string

serverpath2

=

Server.MapPath(

"

images/

"

)

+

System.DateTime.Now.ToString(

"

yyy-MM-dd-hh-mm-ss

"

)

+

Session.SessionID

+

filename;

                FileUpload1.PostedFile.SaveAs(serverpath1);

                ImageThumbnail img

=

new

ImageThumbnail(filepath);

                img.ReducedImage(

0.4

, serverpath2);

//

0.4表示缩小40%

               img.ResourceImage.Dispose(); //这里要释放一下

               

this

.lb_info.Text

=

"

上传成功！

"

;

            }

        }

       

catch

(Exception error)

        {

           

this

.lb_info.Text

=

"

上传发生错误！原因：

"

+

error.ToString();

        }

    }

}