Since updating to macOS Sierra my Photos library will show some bad thumbnails that are either completely black or have black "bars" on them. Here is just one example that I managed to screenshot (see: top left photo).
If you come across this issue it is pretty easy to fix.
- Right-click (or, control+click, or two-finger tap) on the photo.
- Choose "Rotate Clockwise" (this will rotate the photo, and regenerate the thumbnail)
- Rick-click on the photo again
- Hold down the Option key
- Choose Rotate Counterclockwise (this will rotate the photo back to its original orientation)
I don't know what causes these bad thumbnails but at least it is relatively easy to fix them.