Pixlr, an Autodesk product, offers three good tools for editing images in your web browser. All three of those tools can be accessed from the same page at Pixlr.com/web.
Pixlr-o-matic is basically Instagram for your web browser. You can crop your images and apply color filters including "retro" filters.
Pixlr Express provides easy-to-use tools to adjust the color, size, and shape of your images. Text overlays and basic image overlays can also be applied to your images in the Pixlr Express editor.
Pixlr Editor is the most robust of the three Pixlr web apps. Pixlr Editor lets you work with multiple layers and filters on your images. Of course, it includes all of the tools you would expect like for doing things like cropping, resizing, compressing, and changing image orientation. Take a look at the video below to see how the Pixlr Editor "magic wand" lets you cut out portions of an image.
Cutting Things Out with the Magic Wand & Lasso Tool from Pixlr on Vimeo.
Applications for Education
Like everything else in school, when it comes to using technology we all have different skills and different needs. Pixlr is a great option for image editing in your classroom because it offers tools to suit all skill levels.