上传图片 并生成缩略图的 例子 C#.net

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;
using System.Data.SqlClient;
using System.Drawing;
//using System.IO;
public partial class Adi_Default2 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    SqlConnection conn = new SqlConnection("server=.;database=aspnetdb;uid=sa;pwd=qwe2323478");
   SqlCommand Mycomm = new SqlCommand();
    protected void Dupload_Click(object sender, EventArgs e)
    {
        Boolean fileup=false;
        string path = Server.MapPath("~/modules/dtimage/");
        string filegeshi=System.IO.Path.GetExtension(FileUpload1.FileName).ToLower();
        string[] allowgeshi ={ ".jpg", ".gif", ".jpge", ".png" };
        if (FileUpload1 .HasFile)
        {
            for (int i = 0; i < allowgeshi.Length; i++)
            {
                if (filegeshi == allowgeshi[i])
                {
                    fileup = true;
                }
            }
        }
        if(fileup==true)
        {
         try
         {       
         FileUpload upfile = FileUpload1;
         System.IO.Stream Filestream = upfile.FileContent;

         //调用Doit函数生成缩略图并放在另一个文件夹里。
         Doit(Filestream,Server.MapPath("../modules/image/"+FileUpload1.FileName));
         FileUpload1.SaveAs(path + FileUpload1.FileName);
         MsgD.Text="上传成功";
         }
         catch(Exception ex)
         {
            MsgD.Text="发生错误！";  
         }
        }
        else
        {
          MsgD.Text="不能上传这种格式!";
        }
    }
    public bool Doit(System.IO.Stream stream, string savepath)
    {
        System.Drawing.Image img = System.Drawing.Image.FromStream(stream);
        int img_w = Convert.ToInt32(img.Width);
        int imgx_h = Convert.ToInt32(img.Height);
        System.Drawing.Bitmap simage = new System.Drawing.Bitmap(150, 150);
        Graphics g = System.Drawing.Graphics.FromImage(simage);
        g.Clear(Color.White);
        if (img == null)
        {
            return false;
        }
        if (img_w <= 150 && imgx_h <= 150)
        {

            /*Graphics.DrawImage(Image img ,RectangleF Rf ) RectangleF为固定大小跟坐标画图片，有4个Int的参数，也有4个float参数的
              的重载，其他重载参见MSDN*/
            g.DrawImage(img, new RectangleF(Convert.ToInt32((150 - img_w) / 2), Convert.ToInt32((150 - imgx_h) / 2), img_w, imgx_h));
            simage.Save(savepath, System.Drawing.Imaging.ImageFormat.Jpeg);
            return true;
        }
        else
        {
            if (img_w >= imgx_h)
            {
                //计算缩放比例,计算出缩放后的高度
                imgx_h = imgx_h * 150 / img_w;
                g.DrawImage(img, new RectangleF(0, Convert.ToInt32((150 - imgx_h) / 2), 150, imgx_h));
                simage.Save(savepath, System.Drawing.Imaging.ImageFormat.Jpeg);
                //g.Save(savepath, System.Drawing.Imaging.ImageFormat.Jpeg);
                return true;
            }
            else if (img_w < imgx_h)
            {
                img_w = img_w * 150 / imgx_h;
                g.DrawImage(img, new RectangleF(Convert.ToInt32((150 - img_w) / 2), 0, img_w, imgx_h));
                simage.Save(savepath, System.Drawing.Imaging.ImageFormat.Jpeg);
                return true;
            }
        }
        return false;
    }
}

-----------------------------------------------------------------

代码在发布的时候我删除了一些 数据库操作的部分，没测试代码的完整性。仅供参考