OS 10.11 Can't access photo library from Finder file upload box
I'm wondering if an application/bug/corrupted setting is causing this. We can test that by trying it on a completely blank slate by creating a new OS X account real quick. 

You can do this over in Users, Systems &amp; Groups in System Preferences. Create a new admin user account (no iCloud necessary) restart into the new account &amp; test there, ideally with the same exact photos library. I would share the one you have, here's a how to about it on the Apple support site.