Hi Mark,
It might not be as convenient as you would like, but it is possible to do this today.
In the blog if you enable excerpts for the feed in settings then in the post edit page you will see a tab where you can manually edit the excerpt, if you don't edit it then an excerpt will be generated from the main content, but if you edit the excerpt manually then it will use the excerpt you have entered. So you can put an image in the excerpt and that image will be included in the feed so you would see it in feed manager.
Hope that helps,
Joe