I think the main problem with D|S is that it's still in beta. It doesn't do everything Poser does (yet). The renders are pretty good, but it's buggy. It's getting better, but when they fix things, sometimes other things get broken. I find it frustrating. On the good side, it's free.

Saving renders can be a bit quirky in Poser. The best thing to do is render to a new window. (It's an option in your render settings, and lets you make larger images than rendering to the main window.) You should then be able to save the render with File -> Save As. If that doesn't work, close the new window containing your render. You will then be prompted to save it.

With what you want to do, you will definitely want to get Victoria 3 one day. She's a higher-res mesh than the figures that come with Poser. It's easier to get a likeness with her than with older characters. And there's a ton of stuff for her out there - clothes, textures, morphs, etc. She can also wear Victoria 1 and 2 clothing, because she comes with a special "legacy" figure that fits the old clothes.

You don't have to buy figures right away, though. You can use Judy (the Poser 5 woman), or even Posette (the Poser 4 woman, who has a lot more stuff than Judy does) to start with.