First up today, a really useful resource for every time you need an image: https://www.needpix.com/
So far I have found it to be fast and fairly accurate in finding what I've asked it for, and the number of images it returns for any search is pretty staggering. Time will tell if I start using it instead of Google Images to find stuff, but so far I like it. It does seem to do a better job of curating images that you can actually use for free.
In the same vein is http://www.findsounds.com/, although it searches by file name and not by tags/keywords. There is a search-by-category, but all in all it's fairly rudimentary. A better source for sounds is https://freesound.org/. You have to sign in to download, but the collection is larger and more flexible.
Finally, Public Domain Review has a great post, A Guide to Finding Interesting Public Domain Works Online. You can learn how to search some great online resources, several of which you've seen here on our Fun Friday Resources series. You can learn about licensing intricacies, including Creative Commons licensing — and it's best to do that so that you won't find yourself the subject of a cease and desist letter. That's no fun.