Photos without a title are common, especially after an import. Photarium already built a descriptive headline for each of them on its own page, from the people in the photo, its first tags and the date it was taken.
The listing pages that search engines read (tags, albums, people, and the like) were still captioning those photos "Untitled photo". They now use the same generated headline for each image's description and caption, so a page of fifty untitled photos reads as fifty different photos. Photos that have a title are unchanged.