When you say included in the rss feed but not in the content then where? We cannot add arbitrary extra fields to the standard rss or if we do they would be ignored by feed readers as far as I know.
We already have the extra field excerpt which can be used like that, ie if you manually edit the excerpt instead of having it auto generated then you can put an image there and you can configure settings so that the rss feed uses only the excerpt thereby putting an image and brief teaser in the feed for use on another page in feed manager. Users would have to click through to read the full post in that case.
Hope that helps,
Joe