Here are a few of my favorite sites that I like to use, and some of you may have seen me demo these at various education conferences.
Pixlr is one of my favorite companies. They offer Pixlr Editor an advanced interface that looks and appears much like Photoshop. Pixlr Express an intermediate editor with a simple "icon" based interface. And lastly, Pixlr-o-matic, a simple kid-friendly filter tool that lets you click simple filter buttons to add emphasis to photos.
Sumo.fm or Sumo Paint is a great online photo editor that has the look and feel of Photoshop. it is a free site but you can create an account to have access to cloud storage and their social network community or upgrade for $9 a year for unlimited storage and Pro tools.
iPiccy's uninviting home page looks like a mess with ads and a huge malware download ad button right at the top, but look for the bright yellow Start Editing button and this is your entry to a really cool and comprehensive online photo editing site. iPiccy has developed a unique interface that allows for some very impressive editing functions. Play with the tools along the left side of the window along with the category options along the top of the window, and within minutes I'm sure you will be amazed.