mheisig the on device wallpaper being only black is expected as the black wallpaper is stock and comes as part of the OS.
I however cannot repeat the problem you are having, tapping 'My Photos' brings up the app picker and I can choose whatever app I want to source from. 🤷♂️
Might be worth clearing any default apps you have set and trying again if the app picker doesn't come up and we will move forward from there.
SYSTEM>SYSTEM>RESET OPTIONS>RESET APP PREFERENCES
Just be sure to fully read the pop up, if you wish to avoid doing anything listed you will have to go through your apps individually.