There are literally thousands of sites offering images. Here are some no-cost or low cost options to get you started.
Liam McKay lists 8 sites that provide free photos that don't look like am average stock photo.
Check out a repository of digital images of the collections of the National Gallery of Art. More than 45,000 open access digital images spanning over 700 years of art are available free of charge for download and use. Any usage rights restrictions (usually there are none) are identified for each image.
Pixabay allows you to find and share images free of copyrights. All pictures are released under Creative Commons CC0 into the public domain. You can copy, modify, distribute, and use these images, even for commercial purposes, all without asking permission and without paying attribution.
The Graphics Fairy has over 4,500 free vintage stock images, illustrations, old pictures, antique graphics, and vintage printables. DIY and craft tutorials are offered as well. All images on the site are assumed to be royalty free images that are in the public domain.
NASA images cover everything from space and spacecraft, to stunning pictures of the Earth and it's climate and people.
Most of the images on the NASA sites are public domain (see the Usage and Copyright link) and can be freely used in student projects.
There are a number of hybrid sites like FreeImages.com that provide both free and for-purchase photos. With over 300,000 free photos, sites like this can provide interesting images for your student's projects.
Keep A Record!
It is highly recommended that you and your students record the source every image and media item used in a project. If there is ever a question, you can show where the item was obtained and that a good faith effort was made to not violate any copyrights.
Stock photos can be a step up from the varying quality of free photos. For around $10 per image (varies by company), you can get a non-exclusive license to use quality images in you and your student's projects.
Shutterstock is an example of this type of site. We've used a number of their images in this web site.
Google image search can be used in a couple of ways.
If you are unsure if an image is in the public domain, drag and drop the image file onto the Google Image search page. The image will be analyzed and matches are displayed. Go thru the list for matches to stock photography web sites.
You can also use Google to find "free to use" images. Do an image search, then click on the gear icon options button (upper right), select Advanced Search, and scroll down to select the appropriate Usage Rights.