looks like the quickest/easiest way to do this is to make a square pape in jpg or png of 2460x2460 pixels, this should give the phone a complete background image regardless of the phones orientation and whether it's in single or dual-screen mode
alternatively you'd have to start making individual images for each screen and for both landscape and portrait orientation which sounds like just the biggest faff/waste of storage possible, though even worse are the 'live' or 'active' papes, which appear to actually require a pair of short mp4 files, one for each screen.
