文件正由另一进程使用的问题

今天在做一个图片删除时遇到些问题,删除图片时总是报"该进程无法访问文件“XXXX”，因为该文件正由另一进程使用。",很明显是资源没释放的原因.

private void GetThumbNail(string strFileName, int iWidth, int iheight) 
        { 

            System.Drawing.Bitmap oImg;    

            
            float tmpRa1,tmpRa2;
            
            oImg = new Bitmap(Server.MapPath(strFileName)); 
            
            tmpRa1 = (float)oImg.Width/(float)iWidth;
            tmpRa2 = (float)oImg.Height/(float)iheight;
            
            
            if(tmpRa1>tmpRa2)
            {
                iheight = (int)(((float)oImg.Height)/tmpRa1);
            }
            else
            {
                iWidth = (int)(((float)oImg.Width)/tmpRa2);
            }
            

            oImg =  ImgResize(oImg,iWidth,iheight); 
            Response.ContentType = "image/jpeg"; 
            MemoryStream MemStream = new MemoryStream(); 
            oImg.Save(MemStream,System.Drawing.Imaging.ImageFormat.Jpeg); 
            MemStream.WriteTo(Response.OutputStream); 
            
            oImg.Dispose();
            MemStream.Close();
        } 

private System.Drawing.Bitmap ImgResize(Bitmap objImage,int iWidth,int iHeight)
        {
            Bitmap newImg;
            Graphics g;
            
            newImg=new Bitmap(iWidth,iHeight,PixelFormat.Format24bppRgb);
            Rectangle destRect = new Rectangle( 0,0,iWidth,iHeight);
            g=Graphics.FromImage(newImg);
            g.DrawImage(objImage,destRect);            
            
            //objImage.Dispose();
            
            objImage = newImg; 
            
            return objImage;
        }

上面我注释的那段加上就没问题了.
我原以为这个无引用的资源会自动释放呢.