Image handler wont work with GUID

Can any one help me I'm tring to make this image handler work with a UserId  that is a GUID

it seems to work fine when the primary key that is int but not guid how would I rewrite

this code

public class ShowImage : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        Int32 picid;
        if (context.Request.QueryString["id"] != null)
            picid = Convert.ToInt32(context.Request.QueryString["id"]);
        else
            throw new ArgumentException("No parameter specified");
 
        context.Response.ContentType = "image/jpeg";
 & ...