HttpWebResponse

I want to download images from the server. When the image doesn't exist, I want to show my default image.

Here's my code:

string url = "http://www......d_common_conference" + "/" + c.id_common_conference + "-MDC.jpg";

try {
    HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
    request.Method = "HEAD";                        
    HttpWebResponse response = request.GetResponse() as HttpWebResponse;
    string status = Response.StatusCode.ToString();                                               

    img.ImageUrl = url;
}
catch (Exception excep) {
    img.ImageUrl = "images/silhouete.jpg";
    string msg = excep.Message;
} 

It works nice, but until the 24th loop, no response, no exception thrown, and my program becomes jammed.

How can I fix this?