All photos and videos on Pexels can be downloaded and used for free, as long as you use them for noncommercial purposes and don't resell any of the imagery. Users take and upload the photos, so attribution is always appreciated so your viewers know about the photographer.
Image searching website that lets you refine your results to make sure that you are looking at Creative Commons licensed images. After you search, look for the drop-down menu that says "Any License" and change it to "All Creative Commons". It'll be towards the top left.
Very similar to Pexels, this website provides user-submitted photos that can be downloaded and used for free for non-commercial purposes. No special permission needed, though attribution is always appreciated!
As tempting as it can be to simply use Google Images for your poster, you need to resist that urge! In order to follow copyright law, you need to use images that are specifically marked as being allowed for re-use by others. If you do not use such an image, you are basically stealing someone else’s property. Follow the "Can I Use That Image" flowchart below to double check your usage is allowed.
The chart's text is small, so you can hold the Control key down and press the + key to zoom in and the - key to zoom back out.
Use the search engines on this page to find pics with a Creative Commons license, and you’ll always know that you're able to use the image. Remember to cite your images and give the artist attribution, even if you're allowed to use them under Copyright Law!
Image credit: This handy chart is from a Lifehacker article written by staff writer Patrick Allan.