cant that be named http://zope/images/header.gif?uid=Alkj9876 then you can ask for the id with <dtml-var uid> there is also some way to get the URL with all the parameters that where used. However I do not find it.

I considered that, but I do not control the formatting. Another system is rendering the HTML. It is being rendered to zope as http://zope/images/header.gif?Alkj9876. And I dont want to render the graphic unless it has an ID. I guess this is almost like a banner system. How do I change the browser's REQUEST for header.gif to redirect to another GIF?