04-11-2019 07:41 AM - edited 04-11-2019 07:46 AM
No quick answer, I'll defer if there's a difference from my Tab4 8 model running Android 8.1 Oreo (which changed the Settings interface). On mine the simplest is probably to go to Android Settings / Display / Wallpaper / pick a Gallery app / pick the photo and tap it, which opens a screen sizing tool and choice to select. Obviously you have to download an image with appropriate resolution into a folder the Gallery app finds - Gallery apps often have direct "set as .... wallpaper" choice in their menus.
(Personally, I use an app called Wallpaper Setter (Bringe) that tweaks images, mainly my own photos, a bit more. Also most of the image search apps, e.g. Image Search Man (SunSunSun) use Google and/or other engines to find images and they can directly set wallpaper.)