Retouch, colourize, add effects, and more
This web-based photo editor is simple to use, and very powerful. Upload pics and apply all kinds of effects - turn photos into paintings, pencil drawings, charcoal sketches etc. Perfect for creating original illustrations for student stories or graphic novels. (Completed photos can be wasily imported into ComicLife on the Mac or Composer on the iPad.)
Use lunapic with iOS devices. The "Picup" iPhone/iPad app (free) allows you to upload camera roll pics to imgur.com, which gives you a URL that lunapic can use to import and edit. Save images back to your camera roll for use with other apps.