I can only add a photo if I have copied to desktop first. My iPhoto library is not accessible. I read that, in the older version of Story Maps, the privilege level was an issue, but I am an administrator, so I don't think that is an issue. Also, I cleared my cache and cookies and upgraded to Catalina from Yosemite (so I skipped a few OSs) in order to try to fix this issue.
I included two screenshots. The first shows the inaccessible photo files. Here the setting is "Custom" files format.
The second screenshot shows that the files appear accessible once I switch over to "All File" format. However, when I click on a photo library, the window closes and I am back to square one in the Story Map.