Close a book-style foldable and the outer screen is long and slim, often taller than 21:9. Open it and the inner panel is close to a square, around 6 by 5 on many models. One image has to stretch across two shapes that share almost nothing, which is why a wallpaper that looks right outside can look oddly zoomed once the phone is opened.
The system usually fills the inner screen by scaling a portrait image until its width fits, then trimming the top and bottom. On a square panel that trim is large, often a third of the height or more, so skies and foregrounds disappear first. Pictures with the subject held in the middle band survive best, and anything important near the upper or lower edge is the first thing lost.
There is also the crease, a faint vertical line down the centre of the inner screen that catches light at an angle. A thin horizon or a face placed exactly on that line makes it more noticeable, while an off-centre subject lets it fade. Most foldables allow separate wallpapers for each screen, and giving the inner one a calmer, wider picture tends to suit how the phone is held open, more like a small page than a phone.
