For me it must have an exact reason which is more than just personal connection or aesthetic pleasure (but nonetheless includes both). I would never choose a random landscape, even if colors and photography are top notch, because it's just random. Instead, since I've been into buddhism in the last year and it's no minor part of me now, then pic related. /// Also fine art, or references to music and cinema, but only if a piece or album or movie really struck something in me, otherwise it's just lifeless, not "mine". Very very rarely I see a wallpaper that fits "my" category with no previous context (I think just a work by Moebius). Also avoid quotes like plague.