Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Turn your Photos into Custom Icons
Want to create a nice icon from a photo you’ve taken, or even from a photo of yourself? Then do the following:
1. In iPhoto, select the pic you’d like to use as an icon.
2. Select the photo and then click Photos, Duplicate.
3. Click the Edit icon to select the part of the photo you’d like to use as an icon.
4. Click Done.
5. Click Edit, Copy, or press Command+C.
6. In Finder, select the file for which you’d like to create a custom icon, and click Get Info.
7. Click the file icon and then paste the previously copied image by clicking Edit, Paste, or pressing Command+V.
8. Close the window to observe the custom icon you’ve just created.