How to use server side validation to ensure image field is not null

I'm having trouble accomplishing the above. Below is all the mark up and code. I'm not getting error messages, but the validate image function is not working (fileupload portion is working ok).
In my attempt below---not sure if it's the simplest or best validation approach anyway---the "UserId" parameter appears during rendering to be empty---but in most methods I have it successfully containing or referring to the logged-in user's identification.
Following is the control that refers to the image field (page = advertisement.aspx):
<asp:Image ID="Image" runat="server" ImageUrl ='<%#Eval("UserId","Image.aspx?UserId={0}") %>'
 
Following is the successful code behind for the page image.aspx referred to above (page = image.aspx.vb):
Partial Public Class Image
    Inherits System.Web.UI.Page
    Public Sub P ...