Gravity-based you say?
As in composed of gravitiational fields? Or some sort of self-maintaining gravitational wave pattern?
I would suggest you go with some gravitational lensing effects as your images.
Such as this: (from the hubble)
There is a gravitational lensing plugin for photoshop linked to on this page:
http://leo.astronomy.cz/grlens/grl0.html
Having invisible ships that distort the background stars to show their shape would IMO, be really cool
Here's some more:
from Google's archive (source page no longer available), and
http://www.fnal.gov/pub/inquiring/matter/
respectively
[ January 07, 2003, 04:48: Message edited by: Suicide Junkie ]