Wallpaper Juggler downloads, automatically cycles high-res images from InterfaceLift
Perhaps you'd rather take the hands-free approach and let an application handle downloading duties. Have a look at Wallpaper Juggler, a tiny, portable app for Windows which plugs into both sites and cycles through the images it downloads. You can specify the interval at which you want changes to occur and even choose to receive a confirmation alert prior to switching.
The tray app also includes two-click access to delete the current wallpaper as well as a link to your preferences and the download task window. Make sure you keep an eye on that window after firing it up - it'll happily download images until you tell it to stop.
Juggler is a nice, basic app -- if you want something with a bit more "oomph," check out previously-mentioned Wally or John's Background Switcher.