What's the easiest way to implement windows resizing that maintains aspect ratio in WPF?

Hi:

I've got a WPF application which displays a bitmap in an Image control.

I would like to be able to resize the window (and the contained image), but I'd like to maintain the aspect ratio.
Ideally, I'd like to limit the main WPF window to being resized by dragging on the window corners (not the edges).

Are there "thumb controls?"
How would I accomplish this?

Thanks,
JohnB