If you move your photos around in the Finder, Shoebox won't know where to find them and will display a broken file icon. To add the photos back to Shoebox, choose File ▶ Add Photos in Folder, and select the folder containing the photos that were moved.

This may result in duplicate photos in both the old and new locations. Choose File ▶ Utilities ▶ Remove Missing Photos from Catalog to get rid of the missing photos.

To avoid this problem, you can move your photos using Shoebox instead.