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Set up a folder name in Files that matches the camera app.
(DCIM>Camera).
Set the camera app to save new photos there.
Then set Proton Drive to backup the photos. It should grab anything new that shows up.
It took me a while to grasp that the Photos tab (in the Proton drive app) is the only place you'll see your uploaded pictures. You won't see them in a file folder with all of your other folders.