How do I export from Compositor? I've set up all my nodes and connected the last noodle to the Composite node, but how do I then export the new image? The Render Result in the UV/Image Editor hasn't changed and I can't see a Save Image option or anything. Am I missing something or do I need to re-render?
It should work as Mr. Zak says, anyway you can create a viewer node in the compositor, then choose viewer in the UV/Image window and save the image.

I connected the composite output node to the original image to have the behaviour you described, maybe that was the problem?