Fix thumbnails one by one
If a picture in your Photos library has an incorrect or corrupt thumbnail, you can quickly generate a new thumbnail by applying an edit and then undoing it. For example, you can rotate the picture:
- Double-click an affected picture in Photos.
- Click Edit in the upper-right corner of Photos.
- In the list of editing options along the right-hand side, click Rotate.
- Hold down the Option key and click Rotate again. This will return the picture to its original orientation.
- Click Done.
This process forces Photos to refresh the picture's thumbnail based on the actual image. Repeat the steps for each affected picture.
If a picture has a missing thumbnail, you can generate a new one by double-clicking the picture to open it in single-picture view.
Regenerate missing thumbnails with library repair tool
If a large number of your pictures' thumbnails are missing, you can regenerate them with the Photos library repair tool:
- Quit Photos.
- Open Photos while pressing the Command and Option keys.
- Click Repair in the dialog that appears.
This process recreates thumbnails for all the master images stored in your Photos library.
Note: If you use iCloud Photo Library with Photos, this action will cause Photos to re-upload all your pictures.