There is the concept of common carrier status in the law. The phone company is a common carrier, for example. If two people use the phone to arrange a drug deal (illegal act), the phone company is not a party to the deal, or accomplice. In terms of content search, it seems that Google is reasonably a common carrier. The image search is a bit of a gray area. Google would lose its common carrier status if it moderated the results. If a human was looking at the links and saw copyright violation, they'd be required to remove the link. They do moderate, sorta, with their paid links. And I think they reasonably try to assure those links are free of copyright infringement.
