Hi , you are right I think there is no granular way to set notifications for only user photo change. But you may want to look at delta query to track user property changes and set an additional filter on "ProfilePhoto" field.
https://learn.microsoft.com/en-us/graph/api/resources/profilephoto?view=graph-rest-1.0
https://learn.microsoft.com/en-us/graph/delta-query-overview