Talk to them twice and day and do all their favors. If you don't want them to change their house; play hide-and-seek (and win), visiting favors (Only between you and them, not getting a villager to talk to them within a amount of time.), fruit favors, and lost item favors.
Always give them perfect fruit for when they ask for just fruit.
Just doing these alone, I've gotten their pictures, it doesn't take much.
Another thing that increases friendship levels with your villagers is to give them their preferred items on their birthday.I use the Thonky guide(see link above) to find out what they like.
I’m not sure how long it takes to get max friendship, but it takes about 3 weeks to a month on average to get a villager’s picture. But luckily, you don’t need to have a maxed out friendship to get one! You just need a high level friendship. ^^
Here’s a really great guide that should hopefully help quite a lot!: https://www.belltreeforums.com/showthread.php?80401-GUIDE-How-to-get-Villager-Pictures
Good luck!!
Haha oof the aquarium houses;;. What I do when a villager’s house gets a bit messy;;, I just mail them really nice (often customised) furniture that they like. So then not only do they put nice furniture in their house, but it raises friendship with them as well! ;7;
I never really gotten their pictures before, but basically like everyone else is saying, give them perfect fruit, customized furniture and bugs and fish that are at least over 1000 bells, like koi or golden stags. If you keep doing this, you should max out friendship before long. Also, you'll know your getting close to high friendship with them when they start coming over to your house unexpected or without inviting them over.
I find sending them letters really helps. If you’re trying to get their photos, I am the type of person who would cheat or become impatient, and time travel to April Fool’s Day in order to get their pictures. I’ve only ever gotten, like, four pictures legitimately. My favorite one was Bones. His was obtained legitimately and we were super close, it just came naturally. I wasn’t even trying, but he pinged me and gave me his picture.