Many are photos taken by amateurs, the rights to which Microsoft purchased:

Creative Director Jenny Lam expanded the search to Flickr and contacted people who took really interesting pictures, asking them, “So, how would you like one of your photos included among the default wallpapers in Windows Vista?” The Flickr artists were excited to be a part of Windows Vista (one of them by an astonishing coincidence happened to be a beta tester), and after the lawyers had their say–because nothing is complete without lawyers getting involved–Microsoft sent the photographers on a commissioned photo shoot.