Many websites incorporate thumbnails which are miniature versions of copyrighted works. In the United States, the legality of thumbnails goes through the four factors of the copyright fair use analysis. I discuss the fair use four factors in the posting, Making Fair Use a Little Less Fickle.
Most United States courts have ruled that using thumbnails qualifies as a fair use if the use is transformative. Your use of a copyrighted work is transformative if your use of the work is creatively different from the way in which the copyright owner used the work. For example, United States courts have ruled that a search engine’s use of thumbnails to create a pictorial index of webpages is a transformative use that qualifies as a fair use.