I was searching google for a solution too but all I found is this thread
But then I remembered the great screencapture app "Skitch Plus" and gave it a try. With 3 clicks you can capture that image again, resize it and simply drag it back into evernote. Just make sure you deselected "Drag out as original size" in Skitch.
But nevertheless a direct resizing feature in evernote would be great!