Using wiki notation to specify bg image in Theme Builder

I would like to use wiki notation to specify the background image for a panel as mentioned here:

http://www.adaptavist.com/display/Builder/Background+Settings

My understanding of that is that I would be able to attach the image to a page and refer to it like this in the Theme Builder dialog:

 
!SPACEKEY:Page^image.ext!

But I have not been able to get this to work. When I look at the generated css, the url for the background image is blank when I try to use this syntax. It works when I use a full url (http://server/image.ext)

Any tips on this would be greatly appreciated.