Could be any of a number of things:
- Privacy settings
- Any added ‘RESN’ tag(s)
- Where in the source the image was added (i.e. the source itself, or the source/citation, or the event the source is attached to)
- A bug
Possibly others too. I would need to see an example, and all associated GEDCOM data for the individual and the source, privacy settings etc, to provide an accurate response.