To Rebuild the Photos.app Thumbnails:
Backup your library before following these instructions.
Delete the old thumbnails and various parts from the library:
Quit the Photos app.
Open Finder, and go to the Photos.app Library, on my system it is called
Photos Library and is found in the
Now right click the library and click
Show Package Contents.
Next delete the folders called
Then delete the files called
Library.apdb-wal from the
Lastly delete the folders
RKVersion from the
Now rebuild the library
Open the Photos app while pressing ⌥+⌘. It will ask you to if you want to repair the library, click
Repair. It will do a repair, then your photos will go through a reprocess/reimport process. Once finished the thumbnails will start rebuilding.
P.S. I tried this on my own Photos library, and it worked.