Daniel Stoeffler (Dan_Sto) developed a way to make a self-standing mosaic by using hinges to open the panels of the mosaic.
As Daniel says, it is a "self-standing 128x70 mosaic representing the scene of the Star Wars, A New Hope movie where Princess Leia records her help call for Obiwan. The mosaic consists of 4 panels connected with hinges so that the panels make an angle of 22.5 degrees."
Here is a view of the back:
I never thought I'd make a large mosaic, if only for the fact that I wouldn't know how to mount it and didn't care enough to try to figure it out. But this makes it look easy and do-able.
You can read more about this mosaic at Eurobricks or, if you prefer French, at Forum SeTechnic.