It seems to do most of the same edits that can be found on Picnik - and it's just as easy! Here's a picture of some flowers that I took last summer -
And here is the same picture after editing -
The basics - brightness/contrast/sharpness
Black and white - with color paint option
Stickers - including arrows, shapes, stars, and speech bubbles
These are just a few examples. There are many more editing features - and they are all very easy. It even offers teeth whitening and blemish fix. This site is super handy. And free! Woohoo! They do offer a premium package (I haven't tried it yet - but they are offering a free trial for that too).
So far, I think this might be the best alternative to Picnik. Give it a try and let me know what you think.