"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.
