I could be wrong, but I don't think it's possible to have the picture centered. Perhaps you will need scripting, idk.
Don't know if you know this, so excuse me if you do, but it is possible to change the size of the picture. If you double click on it, a window will pop up and you can make the picture either smaller or bigger. You should try to experiment with it a little, find out what looks best. Size can make a huge difference. And a little advice, if you can't figure out, I'd suggest you just put it away for a while and simply focus on the story. It's easy to get lost in details.