User VanSoaked at DevianArt has gathered together a nice collection of over 70 high quality wallpapers, specifically cropped for your Windows Phone's lock screen.
Most of the pics fall into the buildings/scenery camp, so no babes on cars, Calvin's peeing on...things, sports or superheros. But they are certainly high resolution, have no color banding and look great on your adult phone (yeah, I went there). You can download all the pics here in one ZIP file. Definitely worth it in our opinion.
Source: VanSoaked's DevianArt page