WELL lets say it's a picture of someones EYES or something *which is what I'd have on my wall >_>* then it'd be horizontal BUT if it was something like...uh...uhm...random anime characters! <_<... then it'd be vertical
I've always preferred vertical over horizontal for some reason. I find it easier to adjust them on my walls. Maybe it's because my flat has rather high rooms so there's more room vertical than horizontal.