I can‘t See photos

[thenthorn]

Hi - I looked for Forum -> Support -> Guides, Walkthroughs & Tutorials, but couldn't find anything relevant on troubleshooting Photos.
I also had a lot of trouble getting Photos to show on the Mac, while an identical library on Linux worked fine. 
PDFs would show but not JPEGs or PNGs.

I double-checked permissions, refreshed, etc. 

I waited overnight, and the photos showed on the Mac!!!