you can add graphics to the folders that contain the .vfd files. just place them in that folder and they will show up as graphics when adding devices. I usually place mine in the Alpha folder since that's the default folder that shows up.

to change backgrounds and other elements, you would have to modify the .vfd file. though I think I tried that on the mx780 awhile ago and couldn't make that work. those graphics might be hard coded into the remote itself and are unable to be changed. not 100 percent on that.