Google Image Search has long made it possible for users to find images based on usage rights, but they are now giving more prominence to that feature within the "Search Tools" search option.
Users can find immediately find images labeled for reuse, commercial reuse, reuse with modification, and commercial reuse with modification (see screenshot at the end of this post). The image seen immediately below, which happens to be hosted on Flickr, was found through Google Image Search this morning.
Google, of course, is far from the only way to find free images with different usage rights. Perhaps the most well known is the search feature provided by CreativeCommons, a metasearch engine that enables users to search for media from multiple sources simultaneously, that they can use for commercial purposes and modify, adapt or build upon. Users can search through data sets from Flickr, YouTube, Wikimedia, SoundCloud, Fotopedia and others (as well as Google Images).
Keep in mind that when using images with more, shall we say, "open" usage rights, it's important to provide attribution in the form of a link or text citation. Be cautious too because there have been instances in the past when images labeled with licenses were not, causing a series of negative issues for the enterprises involved.