Pixiv is a Japanese community for artists, primarily focused on illustrations. Pixiv requires (free) registration to browse the site, but is available in Japanese, Chinese, English, Korean, Russian, French, and Thai.
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Pro High quality fan art
Much of the content on Pixiv, due to its origins as a Japanese site focused on illustration, centers around anime, manga, and video games. As a result, it's very easy to find a good amount of quality fan art for popular franchises suitable for use as desktop backgrounds and the like.