I just stumbled upon this article about the top 10 Open Source Photo editors comparable to photoshop. This one stood out to me in the list. A web based Photoshop alternative. At first glance it seems to have many of the basic Photoshop features. Many of the photography gurus will tell you get it right in the camera first, if you do that then the basic editing features that Pixlr Editor offers is all you need. Add that to the fact it is web-based and free how can you go wrong. I plan to do a full review in the near future until then check it out for yourself at the link below.
This is a web-based photo editor with a layout that looks a bit like Photoshop CS. It has good an amazing interface and arsenal of features considering that it is a free web application e.g. layers, filters, magic wand, styling, texting, editing and a lot of effects. It does not require signup or anything and you can start editing a file in seconds.