The resolution seems unimportant, when i did the images for my friends factions i used thier facebook profile pictures, now my mate chris had a very small image, though the end result came out very well (it was him at the sealed knot, a re-enactment group so it gave him that steam punk pirate appearence) on the other end was my mate wayne whose image was massive (very expensive digitial camera) both images worked fine.
Aspect ratio...hmmmm on that im not sure trial and error seems to be the key, you would need the devs to weigh in on that.
hope the vid helped though
