The decision overturns an earlier ruling which found in favour of porn firm Perfect 10 which objected to the use of its images on Google's pages.
A panel of three judges said the lower court had got it wrong and the use of a thumbnail could be considered "fair use" under copyright law.
However, Perfect 10 was pleased to hear that the court also found that Google might be liable if its thumbnails pointed to images hosted at sites that did not hold the relevant copyright.
"We cannot discount the effect of such a service on copyright owners, even though Google's assistance is available to all websites, not just infringing ones, " the unwigged ones announced.
"There is no dispute that Google substantially assists websites to distribute their infringing copies to a worldwide market and assists a worldwide audience of users to access infringing materials," they said in their judgment.
Somewhat unusually, both sides in the dispute expressed satisfaction at the verdict.