Referencing Image files in different workspaces

Hi, I am trying to reference a file location using an xfbr string in a image component.

When the image component is configured like this:

File Source Type - Workspace File

Url or Full File Name - <FileName.png>

This works to reference the other workspace.

However when I use an XFBR String Service to return the exact same <FileName.png> it does nothing.

I have also tried <Workspace.FileName.png> & <Workspace.MaintenanceUnit.FileName.png> without luck. 

The idea is to conditionally return different image files.