Jigsaw Planet has thousands of free jigsaw puzzles created by users. It's not necessary to open an account to play these free games, but there are advantages if you register, such as being able to save partially solved games to finish at a later date or the ability to sort puzzles into different albums which will help you organize your favorites.
You can also create your own jigsaw puzzle from your own photos. You can create a puzzle from a subject topic that you are studying/teaching and then link it to a QR code. Students could solve the puzzle to answer a certain question, etc. There are many ways to incorporate this into the classroom and students love to solve puzzles!
To learn how, here's a step-by-step tutorial that shows you How to Make Your Own Jigsaw Planet Puzzles.