Get File Url from a Lookup Column REST API

I have a lookup column that correlates to a background image in my presentation layer. Is it possible to expose the URL to the actual file via the $expand mechanism using REST?

Example REST URI

http://ift.tt/1D36Oi5

If I do a separate REST call to the actual Document Image list there are a couple properties that look useful, the most useful being ServerRelativeUrl. But I get an invalid request error trying to $expand on this property. See below.

The query to field 'Document_x0020_Image/ServerRelativeUrl' is not valid.

I'd very much appreciate any help with this issue. Thanks.