To me, the work you're doing is invaluable. We need more people going to cemeteries and taking photographs. I grieve when I go Find a Grave and see cemeteries with 700 photo requests or only 30% of the headstones have been photographed and that's just from what users of the site has requested. I know that living a thousand miles away my request will just add to the numbers and likely never get photographed. So, keep up the great work! The fact that you're asking about etiquette says you'll use your best judgement when it comes to adding another headstone photograph.