If you are seeing photos in BG cemeteries that are the same as the people you have put into your Find-A-Grave memorials, this can not be done. Once we as photographer volunteers take a photo in the BG app and upload it to the website, it becomes the "copyright property of BG" and they do not allow sharing with other sites. That is one of the terms and conditions.
It may be that the original photographer kept a copy of the photograph for their own use and if you can contact them directly they might be able to share it with you and post it directly to your F-A-G memorial. Worth a try.
You just can't copy the one from BG and paste it to F-A-G. If you did try it would have a BG watermark/copyright stamp on it anyway. So not a good idea to try.