The Thumbnails utility at Admin>>Media>>Thumbnails will create thumbnails for all media records that refer to files with .jpg, .jpeg, and .png extensions. (It can take several minutes to run.)
At that point, any media records
a) whose filename has a .jpg, .jpeg, or .png extension, AND
b) without a thumbnail value
represent files that are missing.
People whose "Living" flag is set, sorted by Birthdate and last name.
Since the results show Marriage Date, people with more than one marriage are listed more than once. If you remove Marriage Date from the "Fields to Display", the report will show you the correct count of people who are marked as Living.
Note that under Admin >> Miscellaneous >> Data Validation, you'll find a report called " Person marked as deceased, possibly still living", which lists people who are not marked as living, but who should be.