I used to go to InterfaceLift, but it's turned into a repository of HDR-ruined hypersaturation. I have a program on my PC whose name I can't remember that I purchased to handle my dual monitor setup (it does that well, too), and it turns out that you can give it feeds of wallpapers to cycle through. So I set it to the most popular of various websites that I trust not to churn out images of scantily clad women draped over expensive cars.
Which isn't to say that I never get those pictures. I just get them sufficiently infrequently to make it manageable.