Picadilo is a free image editing service that offers a lot of useful tools. To use Picadilo you do not have to create an account on the service. To get started just upload a picture and start editing. After your image is uploaded you can re-size it, crop it, add filters, touch-it up, add text, and put digital frames around your pictures. When you're happy with your edited image you can download it and or share it on Facebook.
Applications for Education
If you're looking for an image editing tool that doesn't require your students to install any software or create online accounts, give Picadillo a try.