An important change here will be to ensure that when people resize the image, by default, the aspect ratio is preserved.
We've had a lot of experience dealing with end users with this sort of thing, and one thing that's sure is they will inadvertently distort the image, and be upset about it, without this as a default. We need to protect them from them self with tools such as this :P
Will Rossiter created some great design for PhotoEditor. I took the HTML/JS/CSS and the results can be viewed at test server (http://test.silverstripe.com/mujma admin/password). For those of you that don't have time to play with it I've provided a screenshot.
All code was commited in r39716.
I'm working on preserving ratio, I think I will implement it in 2-3 days.