How do you save the wallpaper on a Chromebook?
When you right-click on the desktop, choose "set wallpaper". You
will then see the custom tab. Choose "+" to add your desired custom
wallpaper. Then, select "choose file" to add your image. Select
that image and open. You can edit the way you want that image to
display (center, center cropped, stretch). After that, your image
will then display on your desktop.