Kozzi.com – photos without borders – is a great website that contains a really huge number of pictures that can be used for about anything. They are free and the user has the right to download and use them without giving any credits. You have to sign up in order to start using the photos but the registration is free.
Here are the categories of pictures that are included:
What is also commendable about this site is that it gives you the option to create a lightbox folder where you can bookmark pictures that can be downloaded or used later. You can also share your lightbox folder to social media (students, collegues etc).
Here are some screenshots of this website: