You can add the photo ID of any image on Wikitree as the primary or background image to any other profile on Wikitree. There's a small box ("Edit photo settings") to add it on the right side of the edit view.
Alternately, you can upload an image to a page, make it the primary photo, then remove the image from the profile (to someone or somewhere else), but it will stay the primary unless you specifically remove it as primary.