<asp:FileUpload ID="FileUpload01" ClientIDMode="Static" onchange="this.form.submit()"   runat="server"/>

在page_load事件后面的代码中

if (IsPostBack && FileUpload01.PostedFile != null)
{
   if (FileUpload01.PostedFile.FileName.Length > 0)
   {    
       FileUpload01.SaveAs(Server.MapPath("~/Images/") + FileUpload01.PostedFile.FileName);   
       imguser.ImageUrl = "~/Images/" + FileUpload01.PostedFile.FileName;
    }
 }