What the least painful way to make photography portraits of sims that I can hang on a wall in game?

"Paint from reference" with the easel, or "Take photo" with phone or camera, work basically the same way. You can make an image of anything happening in the active sim's view at that moment. You need a certain amount of painting skill for the option to come up on the easel. For the photo, you don't absolutely need photography skill, but the more skill you have the easier it is to aim the camera and the photos will look better too.

You can rotate the view before you make your final selection of image, but you can't move the camera. So you need to have the active sim standing where you want the point of view to be before you start.

To photograph a single sim you can use "Take photo of..." and the sim will automatically pose for you. Getting the right multi-sim pose can be tricky as sims tend to wander off :) And with the easel, it costs 100 simoleons per attempt, so trial and error can add up. I find it least painful to turn off autonomy and/or to have sims seated.

I don't have moschino, but I understand it came with some pose markers which could be helpful if you want to have more posed-looking pictures.

The photo studio is fun but I like it more for parties since the poses are all kind of goofy. And it's limited to 2 sims in the photo (unless moschino changes that). Maybe having it there at the wedding reception could be fun, but now that we have the photo booth that could be even better.

Whatever you choose, best to do some practice before you're at the wedding.

Get the Moschino SP and make fantastic photos with the tripod.

If you don't want to take the time to skill in photography, you can download a photographer sim from the gallery. Have them take whatever pictures you want, and resize the photos as desired with the keys. I suggest having a dedicated photo session that's not during the wedding event. Turn off autonomy, cheat their needs if necessary, get some nice photos, and when you're done, quit without saving.

If you want photos of the actual event, you might want to add a photographer to your household so you can control them. Pause, teleport them where you want them, and have them take a picture. If your Sims have enough money, just take picture after picture, and discard/sell the ones you don't want later. (Edited to say: Remember that it takes a few seconds for a Sim to teleport and another second or so for the picture to be taken, so don't wait until the perfect moment before teleporting).

As for painting from reference, it's going to have a streaky filter on it to make it look like a painting. The Sims won't be featured in any real detail. I think you can put filters on photos, so you might want to start there and experiment with the filters. If you really want a painting though, a painter with high enough skill can take a single 'mental' picture and paint from reference later.