What's a good online image creator/editor? Free if possible...

I use pixlr.com a lot. Its not quite Photoshop but its pretty close for an online program. And now that its been purchased by AutoDesk (Makers of AutoCAD software) it has a big budget for support and adding features.

I've used SumoPaint a few times and can definitely recommend it.

There is a free version which you can use in your browser, and my favorite thing about it is all the cool unexpected tools and effects it comes with.

It has a fun "symmetry tool" which is great for drawing wild patterns, and you can create awesome looking gradients. 

They have a whole YouTube channel if you wanna check out tutorials etc, but here's a quick video to highlight some of the things you can do with it: