Image TV Custom Directory

How can I make the file browser show a specific directory when the user is selecting an image for a template variable? For example, show directory /assets/images/banner/ when selecting the image for the banner template variable.

Thanks,
Jamie

Bump?

I realise this is an old topic, but for future reference of searchers like myself, here is the solution if you are using

Revolution:
http://rtfm.modx.com/display/revolution20/DIRECTORY+Binding

Evolution
http://rtfm.modx.com/display/Evo1/%28at%29DIRECTORY