Mary: It is possible but not particularly easy nor desirable. Even if they were taken with a GPS tagged regular camera, they can not be uploaded by using the BG smartphone app. They can only be added as Supporting Records, a category which was created mostly for other kinds of documents like cemetery lists, death certificates, etc, and older images of a stone to document what was there in the past (especially important in old historic cemeteries where the stones are very weathered or damaged with parts missing.
If the cemetery where your photos were taken has already been done within the BG app, you might want to see if some of your images would really qualify as Supporting Records,
The beauty of the BG system is the attachment of the GPS signal to each image, and the placement of that as a pin on a Google base map, so we can see approximately where a grave is located. I have not uploaded photos as supporting records but my understanding is that they will not have GPS attached and may not be lilnked to the primary record which would be created when an original BG image was transcribed.
There are lots of discussions on this topic out there in the archives of the Community if you want to read others comments or pros/cons.