To use custom background you need to load your own images inside customBackground folder located on Albedo ES Theme root directory. Your images need to be named after the desired videogame system. The name must the same used by your Operational System. Albedo ES Theme was made for AmberELEC (but is compatible with other systems running EmulationStation) and use the same names from tag <theme></theme> found in es_system.cfg.
For example: If you want to add a custom background for Sega Genesis, the image need to be renamed as genesis.png, for Final Burn Neo, fbn.png . Albedo ES Theme accept images in JPEG, PNG and WEBP formats.
Please, use the images inside assets/systems/ as a reference.
After placing your images inside customBackground, enable custom backgrounds in your device settings.
On AmberELEC, press Start to open system menu > Navigate to UI Settings > Change Theme Set to es-theme-albedo > Theme Configuration > Change Theme Background to Custom. Exit to apply changes.
If you prefer using a tool for that, you can use Albedo Wallpaper Cropper. AWC is a simple tool and easy to use. You can access its README to learn step by step how to prepare images and load them to the theme folder.
If you're concerned about privacy, this web app will never collect or send any information anywhere. The images loaded are saved locally on the browser database so you won't lose your progress. As an web app, it can be used offline after the first load.