Full text search does not search in image metadata
A user tagged his/her pictures (eg with Shotwell or Gimp) so the pictures have Exif (or other) metadata. Now doing a full text search with Nautilus for text in the Exif metadata does not list the pictures.
When Nautilus does full text search, also search for picture metadata (at least keywords but needs to be defined which tags should be searched in).
Benefits of the solution
It's easier to find pictures which are tagged with metadata
Users might not understand why a picture is in the search results. But it is already possible to show the picture metadata (via file properties->Image) in Nautilus. So it shouldn't be very hard to understand.