If you have changed the default wallpaper of RemixOS, it is not simple enough to get it back because it does not appear in the gallery. For example, I also changed my wallpaper as you can see in the screenshot below:
To get the default wallpaper back, follow the steps below:
1. Download the “Wallpapers” app from Google Play Store. You can see its icon below:
2. Open the app and then open the “On device wallpapers” folder.
3. Click the first wallpaper.
4. Click on “SET WALLPAPER”.
5. You will get the default wallpaper back.