In the Proton drive app you should be able to select the options for the folder with your photos in (three vertical dots to the right of the folder name) and select “make available offline”. That should download the photos to your device - however I don’t know if that makes them available to other apps to use (as in properly restores them)
Edit: it seems at present the photos folder is separate to all other backups and is not available via the desktop app either. Apparnently this access will be “coming”. So I can’t see a way to restore your photos. Maybe someone knows an alternative route.
In the Proton drive app you should be able to select the options for the folder with your photos in (three vertical dots to the right of the folder name) and select “make available offline”. That should download the photos to your device - however I don’t know if that makes them available to other apps to use (as in properly restores them)
Edit: it seems at present the photos folder is separate to all other backups and is not available via the desktop app either. Apparnently this access will be “coming”. So I can’t see a way to restore your photos. Maybe someone knows an alternative route.