public class MyHandler : IHttpHandler
{
    public RequestContext RequestContext { get; set; }
    public void ProcessRequest(HttpContext context)
    {
        int imageID = int.Parse((string)RequestContext.RouteData.Values["ImageID"]);
        ...
    }
}

public class MyHandlerRouteHandler : IRouteHandler
{
    public IHttpHandler GetHttpHandler(RequestContext requestContext)
    {
        return new MyHandler() {RequestContext = requestContext};
    }
}