Where to get photos that you can use for free:
Public Domain PicturesJust by typing public domain picutres/photos in google search, it'll generate loads of sites where you can download photos for free
PublicDomainPicutres.net, you can buy the contributor a coffee by donating if you really like the photos.
Creative Commons search, you can search for images you'd like to use or images you can alter.
Wikipedia Commons has a large database of good quality photos, for pretty much a lot of things a teacher is looking for. There are also videos for the search term, e.g. here's a video for jellyfish.
Flickr let everyday user share their photos, users can choose to share their photos to the world when they upload them. Here's a summary of all the creative commons licences. There's a lot of historic photos as well in The Commons.
What about protecting your image? There's many programs/apps that let us add watermarks, copyright information to our image.
Lets you edit photos, add watermarks, quotes to photos and make a collage at lightning speed.