As long as that AC adapter has a plug that will fit into the Duo (I believe you need a 6.5mm one) and is the correct polarity (this is very important), it's fine. I'm using a 9V adapter on mine, and it works brilliantly. The Duo actually only uses 7V internally, but the AC adapter that was packed with it provides more so as to make absolutely certain that it always has adequate power at all times. With a more modern AC adapter, there should be less variance in the power it supplies, so I wouldn't worry about it. Just make sure that the plug (that goes into the Duo) is positive center. Negative center will fry your Duo.
Also, the screenshot looks fine to me. Unfortunately, your games are going to look weird on an LCD, and there's not really anything you can do about it. I can't tell if you're running it in widescreen or not from the picture, but if you are I would suggest switching to pillarbox (i.e. 4:3, the old TV aspect ratio). It'll look a lot better that way.