I have a few answers for you.
1. Tumblr is a good place to start, for example
here or
here.
2. Yes you can, you need to teleport or summon them with the "Teleport Any Sim" statue - it's a mod that is needed for pretty much all poses anyway - if you don't already have it, it's linked in the first page mentioned under 1.
3. Not unless you add them to your household. You need to have an active Sim take a picture if you want to keep that picture. You could make a separate household called "photographer" and add them to your household whenever you need pictures, then transfer them back to their own household once you're done.
As for other tips, pay attention to the lighting. It might be a good idea to add one or more subtle saucer lights above the spot where you want to take the photo. Also, don't add too many pose packs to your game at once, it'll be hard to find the ones you want (and it takes a lot of scrolling). If you do share posed pictures (e.g. on a blog), it might be useful to keep track of where you got them from, in case anyone asks where they can find the post in question. Other than that, just have fun (and don't spend too much time looking for poses ;) - it can take up quite some time).