I think there are two sides to your question:
- if an app has root access to your pod (you are logged in to the app), then how should it find and list the photos on your pod, and allow you to pick a photo to add to a post.
- should apps even have root access to my pod?
For ‘app’, read ‘third-party web app’, for instance https://pheyvaer.github.io/solid-chess/.
Answer to the first part is probably along the lines of: check type registry to find photos, or just show a simple folder tree and let the user click on folder names to traverse it. Displaying a folder tree is a solved problem, as @rimmartin commented.
Answer to the second part is maybe more interesting, and something a few of us are currently looking for, I think. 
I’m working on an answer here, maybe we can brainstorm about that together?