A picture is worth a thousand words, but it might also be worth a thousand dollars if your school gets hit with a copyright violation claim. This happens to schools every year as students or teachers inappropriately use an image that they do not have the rights to.
Thankfully this does not have to be the case as there are loads of high quality pictures that can be used in school projects without any licensing concerns. These can include images that are released under creative commons, or are in the public domain, or simply are copyright-free. Of course depending on the license, it may still be necessary to cite where the image came from to provide proper attribution (and it can be a good practice to do even if not required).
There are many sites and tools that can help educators and students find free photos, clipart, icons, and more. See below for a list of many of these, and be sure to share your suggestions for other resources to add to this collection.