How I normally get a pic is wait for them to give me a request, requests are good for pics. However you must have a high enough friendship with the villager to receive a pic as you know. A good way of making them ping is loading up the game, ran past any villager till one pings, if not the right ping save and quit the game and do the same thing over again. Once you get a good request, such as perfect fruit, signatures, get a rare bug or fish, or my favorite delivering them a present. Once I get a request, save the game. Once you got the thing you need for that request, save the game. Then find the villager and give them whatever they needed and if its not the pic they hand you quit the game without saving. Don't worry it won't harm your game, its just Resetti might get a little mad... Just keep repeat this, giving them what they want till they give you the pic, it normally takes 5 - 20 times for me till they hand it over~ Hope that helps.