In thumbnail view, Shoebox shows the same icons as in the Finder. By default, Shoebox creates thumbnails for image files that don't already have them (you can change this in Photos preferences.)

Shoebox can't create thumbnails for files that you don't have permission to write to, or that are on a locked volume.

To update the thumbnails for the selected photos, hold down the option key and choose Photos ▶ Update Thumbnail (⎇⌘T).