There’s a commonly-held myth that any image found in Google images is fair game for anyone who wants to use it. In reality, most images are copyrighted by default and usage generally requires permission – especially for commercial use.
Google’s Matt Cutts has tweeted that you can now filter images by usage rights. If you want to find images you can use commercially, for example, just do your search, click Search Tools and then select ‘labelled for commercial use’ from the pull-down.
It’s not immediately apparent how Google identifies the permissions associated with an image. We’ve reached out to Google and will update when we have a response.