Customized Templating popup windows

tobias

I'm trying to find out how to customize the templating popup windows.
For instance the Image popup from the VisualFormat toolbar:
I don't want the workarea to be the root directory of the shown folder structure, but a specific directory called images instead.
The root directory is handed over in a GET parameter called topdir, which by default has no value. If I manually change this value to images then the image browser starts in this directory which is exactly what I want it to do by default.
How and where can I set/change the default values for this (and other) parameters?
Has anyone an idea?