How Do I Make Image' Name Validation

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C#

I need a way of making an image name validation so that a message is sent every time a user sends/posts an image with a name that already exists.

The problem is people posts the same images all over and again because the is nothing that stops them from doing so. Now i need to find a way of putting a stop to that so that every time a user sends the same image a Message Box shows up and say "This image already exist" so that the user may know that now i need to change and post another one. Please help!

Here's my code:

public class ImageHandler : IHttpHandler
{

    public void ProcessRequest(HttpContext context)
    {

        {
            System.Data.SqlClient.SqlDataReader rdr = null;
            System.Data.SqlClient.SqlConnection conn = null;
            System.Data.SqlClient.SqlCommand selcmd = null;
            try
            {
                //string path =""
                conn = new System.Data.SqlClient.SqlConnection
              (System.Configuration.ConfigurationManager.ConnectionStrings
              ["LiveAdsConnectionString"].ConnectionString);
                selcmd = new System.Data.SqlClient.SqlCommand
             ("select Image from Adverts where AdvertsID=" +
              context.Request.QueryString["id"], conn);
                conn.Open();
                rdr = selcmd.ExecuteReader();
                while (rdr.Read())
                {
                    context.Response.ContentType = "image/jpg";
                    context.Response.ContentType = "image/png";
                    context.Response.WriteFile("/Images/" + rdr["Image"].ToString());
                }
                if (rdr != null)
                    rdr.Close();
            }
            finally
            {
                if (conn != null)
                    conn.Close();
            }

        }
    }
    public bool IsReusable
    {
        get
        {
            return false;
        }