<table style="width: 100%">
<tr>
<td>
<asp:ValidationSummary ID="ValidationSummary1" runat="server" />
<br />
<asp:FileUpload ID="FileUpload1" runat="server" />
 <asp:Button ID="btn_upload" runat="server" OnClick="btn_upload_Click"
Text="Upload" />
  
<asp:CustomValidator ID="CustomValidator1" runat="server"
ControlToValidate="FileUpload1" Display="Static"
ErrorMessage="You should only can upload image file such as files with .jpg or gif extension"
OnServerValidate="Image_validate">*</asp:CustomValidator>
</td>
</tr>
</table>

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.IO;
using System.Drawing;
public partial class practice_FileUpload : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void btn_upload_Click(object sender, EventArgs e)
{
if (Page.IsValid)
{
string path = @Page.MapPath("User_Edit.aspx").Replace("User_Edit.aspx", "") + "Documents//";
string s = path + Session["UserName"].ToString();
if (!System.IO.Directory.Exists(path + Session["UserName"].ToString()))
{
System.IO.Directory.CreateDirectory(path + Session["UserName"].ToString());
}
if (FileUpload1.HasFile)
{
FileUpload1.SaveAs(Server.MapPath("~/Seeker/Documents/" + Session["UserName"].ToString() + "/" + this.FileUpload1.FileName));
}
}
}
protected void Image_validate(object source, ServerValidateEventArgs args)
{
string fileExt = Path.GetExtension(FileUpload1.FileName).ToLower();
string fileName = Path.GetFileName(FileUpload1.FileName);
if (fileExt != ".jpg" && fileExt != ".gif")
{
args.IsValid = false;
}
}
protected void CustomValidator2_ServerValidate(object source, ServerValidateEventArgs args)
{
Bitmap bmIP = new Bitmap(FileUpload1.PostedFile.InputStream);
if (bmIP.Width > 100 | bmIP.Height > 100)
{
args.IsValid = false;
}
else
{
args.IsValid = true;
}
}
}

<httpRuntime
executionTimeout = "110" [in Seconds][number
maxRequestLength = "4096" [number]
requestLengthDiskThreshold = "80" [number]
useFullyQualifiedRedirectUrl = "false" [true|false]
minFreeThreads = "8" [number]
minLocalRequestFreeThreads = "4" [number]
appRequestQueueLimit = "5000" [number]
enableKernelOutputCache = "true" [true|false]
enableVersionHeader = "true" [true|false]
apartmentThreading = "false" [true|false]
requireRootedSaveAsPath = "true" [true|false]
enable = "true" [true|false]
sendCacheControlHeader = "true" [true|false]
shutdownTimeout = "90" [in Seconds][number]
delayNotificationTimeout = "5" [in Seconds][number]
waitChangeNotification = "0" [number]
maxWaitChangeNotification = "0" [number]
enableHeaderChecking = "true" [true|false]
/>