You could try adding these photos as supporting records at https://billiongraves.com/add-record?batch=true&record_type=headstone#/. Before you do so, you should check the FindAGrave Policies,Terms of Service, etc. to see if there is there is any policies that forbid you to upload headstones from their site to BG.
You should know that in most cases, you should avoid adding existing photos or photos not taken with the BillionGraves app as primary records. Supporting records could be harder to search in the BG System. BG is meant to have primarily records from photos taken with the app, this is because they want a GPS coordinate of the headstones connected with the record.