The easiest way to find the image you're looking for is a Bing/Google image search.
Did you know, though, that Bing and Google (and likely the other players) actually do their best to index the licenses associated with the images they crawl?
In a Google Images search, you perform a search and then hit Search Tools and then drop down the Usage Rights. As to why Google gives us astronaut Neapolitan and strawberry waffles... well, I can't speak to that, but I'm sure those algorithms are really hard to write so we should have grace.
In a Bing Images search, you perform your search and then drop down the License list.
In either case, your search results are going to be severely limited, but you're going to be able to sleep well at night knowing that you didn't just rip an image off of someone's site without a second thought only to reproduce it on your own and in the process breaking the law, ruining your name, and on and on.
I would imagine that this sort of search filter is highly dependent on the presence of some good metadata, so in order to be on the safe side, I still recommend you click through to the site where the image was found and do the leg work to figure out if it's really in the public domain.