Are you familiar with Find-A-Grave or BillionGraves? Both websites allow you to post photo requests for headstones.
For Find-A-Grave, if the person(s) don't have memorials already, you can create them.
For BillionGraves, you make the request on the cemetery main page. If the cemetery hasn't been added yet, you can add it.
Other possible options would be to see if there is a local historical, genealogical, and/or hist/gen society. Other places I check when looking for headstone photos are the county's official website, GenealogyTrails, Interment.net, and USGenWeb.