As an art teacher, I have been collecting websites that provide good-quality copyright-free images for many years. Please note that copyright-free means that you don't have to pay or ask to use certain pictures but you should still give credit to the photographer. Some sites have Creative Commons licenses and those should be attributed as requested. Here is an excellent article about what permissions you need to have: https://www.cultofpedagogy.com/online-images/
I also want to warn you about a potential scam where someone insists that you are using images that aren't copyright-free and insists that you link to their website. The payoff for them is that the more pages link to a site, the more potential for moving up in Google ranking or other benefits. So if you get a letter about this, follow the advice in the link.
Sources for Images
Unsplash and Pixabay and Morguefile are some of my favourites!
Art Institute of Chicago - https://www.artic.edu/collection - 1,000s of images under CCo license (Public Domain Dedication, no Copyright)
Paris Musee - many of the digital images in their collection are now in the public domain. http://parismuseescollections.paris.fr/en
Bing Images - use search features to locate copy-right free images
Creative Commons Search tool - https://search.creativecommons.org/ - choose your search location and keywords
Every Stock Photo - http://www.everystockphoto.com/ - is a search engine for public domain and Creative Commons licensed pictures.
Flickr.com - Enter your term in the search bar at the top. Select Advanced Search, Creative Commons, then once you pick your image, open original size or large size. You can also search on Tags only. You can also directly search Commons.
4 Free Photos: https://www.4freephotos.com/
Free Reference Photos for Artists - this is a Facebook group with nature photos: https://www.facebook.com/groups/1628011010653357/
500 Pix: https://500px.com/creativecommons - this is the Creative Commons page of 500px.com. Not all the images are free for use.
Getty Images - https://www.gettyimages.ca/ - for most of images, watermark has been dropped and a footer appears at the bottom of the pic with credit.
Google - Use the Advanced image search feature to specify Labeled for reuse (with modifications) when searching.
Icons for Everything - https://thenounproject.com/
Library of Congress - https://www.loc.gov/free-to-use/
MorgueFile.com - Perform your basic search, then refine by size. Check the accompanying licence to make sure of how you can use the picture. Not all are copyright-free.
Open Clip-Art: https://openclipart.org
http://www.photosforclass.com/ - download attributed, Creative Commons photos for school
PhotoPin: http://photopin.com/ - search creative Commons photos
Picjumbo - https://picjumbo.com/ - free stock photos
Picsearch - https://www.picsearch.com/
Pixabay - http://pixabay.com/en/ - free images. Caution (don't use sponsored images on top line). If you aren't logged in, then you will be limited to certain images and sizes.
http://images.superfamous.com/ for some beautiful, royalty-free images (just give attribution)
Paint my Photo - https://www.pmp-art.com/ - a social network of copyright-free images uploaded by artists. Photos of artwork are called Artwork. Photos that you can paint are called Reference.
Photo Pin - http://photopin.com/ - Creative Commons photos
Pond 5: https://www.pond5.com/free - includes historical photos
Pexels: https://www.pexels.com/ - free stock photos
Public Domain Pictures - https://www.publicdomainpictures.net/en/
Republic Domain: http://republicdomain.com/
Stockio - http://www.stockio.com - thosands of files for personal and commercial use without attribution
The Noun Project - https://thenounproject.com/
TinEye: http://labs.tineye.com - search by colours! Put the word "free" in the tag box to help find free images (double-check!)
Wellcome Collection: https://wellcomecollection.org CC licensed images from historical to contemporary. Good for science and history.
Wikimedia Commons - https://commons.wikimedia.org/wiki/Main_Page - database of Creative Commons images, sounds and video clips
How to Use the Creative Commons Chrome Extension