Make sure the photo contains the © copyright symbol and the words "copyright (year)"
Then you can alert the site publisher there's infringed copyright material on their site, and they are obligated to take it down.
I'd still alert backpage and I'm guessing they'll take it down.
Originally Posted by version1955
The photographer owns the copyright unless he or she has relinquished the copyright to another party (such as the subject in the photo). The image does not need to contain the copyright symbol for it to be copyrighted. The EXIF data contained in the image file can indicate that the image is copyrighted. The owner of the copyright is the party that needs to ask for the image to be removed.