Looking for good quality free images to use in your presentations or posters? Try Pexels which has some excellent photographs. I searched for both books and libraries and got some really good stuff on both searches. Pexels also adds 10 new images a day. Each photograph comes with attribution data and a clear indication of what you can do with the image, so you're never in doubt.
Alternatively, try Unsplash, which also has some great images, which are entirely free to use however you want, including commercially. The photographs are of excellent quality and really large, which is just what you need if you're going to be messing around with them.
Stocksnap is a more traditional type of image search engine, but again all the images that it retrieves are free to use with no attribution required; though personally I think it's always nice if you do that. It doesn't have the same quantity as Unsplash, but the images are all good quality.
If you're more interested in videos, try Coverr. It's got a very poor search function - mainly a few basic headings, and then you have to scroll through to find exactly what you want. It didn't particularly excite me, but if you're in need of a bit of stock footage for a project, this is a good place to come.
XStockVideo is another video search site, but some of the video may have commercial use restrictions on it and are available for a fee. Really didn't find this site terribly helpful when it came to searching, with only a few major subject areas, so it would take a lot of browsing to find what you need.
Videezy is slightly better in that it does have a search function. Some of the videos are of really high quality (ie. 4K) and there's a nice selection. I found myself really drawn into this site, since it has some really interesting video clips - mostly about 30 seconds long, so it's always a case of 'oh, just one more!'.