Due to some iOS restrictions, setting the wallpaper may not feel very straightforward.
Here's how to do it:
1. Find the image you like in Artpaper. Press the "Save" button on the lower right.
2. Allow photo library access. Artpaper needs it to save the image.
3. Open the Photos app, find the freshly saved image. Press the "Share" button on the lower left.
4. Select "Use as Wallpaper".
5. Tap "Set"
All done! You now have a beautiful artwork set as your wallpaper.