There is no doubt that the most popular wallpaper out there was Bliss, XP’s default wallpaper. The image depicts rolling green hills set against a background of blue sky and a few clouds.
The image was an actual photograph, not a computer rendering, taken on a January day in the late 90s. The image was taken by the professional photographer Charles O’rear, who lives in St. Helena, Napa County and works for the digital-design company High Turn. The image was taken from a hill in Sonoma County, California, which the author always passes on his way to work. The hill was originally a grapevine plantation, but during the previous summer, a great fire burned all of the bushes, turning the fertile ground barren for a while, to be replaced by grass later.
The hill was revisited and rephotographed in 2006. You can see the photo here.