I've had an area on the Shambles website to help Schools find free photographs, clipart and graphics for several years now. www.shambles.net/image_search/
It includes a link to a large list of (safe) websites, plus a link to a tutorial on 'Searching the Internet for Images'.
It also includes a search box to 'picsearch' which has a compulsory 'Family Filter' built in ... making it a pretty safe option for schools and families to use.
Now I have just set up on the page a second search facility that will go through 25 different websites that host free-to-use images and photographs ... although you are advised to check any restrictions that might have been imposed on individual pictures.