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.
