I personally headcanon that Hanma has never really had a reliable roof over his head, so he spent a lot of his years couch surfing and staying in motels and sometimes even having to sleep on the street. It never really bothered him per say but he's secretly sentimental so he would always take pictures of the places he stayed and the people he stayed with (he definitely has a bunch of photos of the inside of Kisakis house and some unflattering candids of Kisaki, even one where he's asleep lmao).
I think that really fits with Hanmas overall chaotic energy and with him becoming a freelance photographer in the final timeline. No rules, no restrictions, just movement and freedom.
This is also why I imagine that when you and him finally start dating, he takes a million candid photos of you and always carries some of them with him. He'll definitely try to get you to go on one of his photography road trips with him because what's better than always having pictures of you on hand? Always having you on hand. He's an ass and a menace to everyone else but you get to see his secret, soft, sentimental side.♡