iPhoto is a great software that imports, exports and edits your digital images. While it is a great option to store your images, sometimes the thumbnails in iPhoto may not show up or just go missing.
In such cases, it is essential to remake them, to restore your thumbnails used for displaying iPhoto. If you encounter same or any other kind of problem, you can always resolve it by using the Photo Library First Aid.
Here's How to Fix iPhoto Thumbnail Not Showing up in Mac OS X:
Step 1: Create a backup of your iPhoto library with the help of Time machine so in case something goes wrong, you have the data.
Step 2: Before beginning the Photo Library First Aid process, quit iPhoto, if it's open.
Step 3: Open Finder → Applications
Step 4: Select the iPhoto app but do not open it.
Step 5: Press the Command and Option keys and simultaneously and double click on the iPhoto app to begin it normally. Hold on to the keys.
Step 6: The Photo Library First Aid screen will appear. From here, choose “Rebuild Thumbnails” followed by “Rebuild option” at the bottom right corner.
Your thumbnails will re-appear. However, do note that this option will only work if the original files are still there. In case the original images are distorted, then you can choose the “Repair Permissions” option to restore them. Similarly, there are other options that the Photo Library First Aid provides. Depending on your problem, you can select and hit “rebuild.”