I finally found the time to dig into this.
The generic <image> is only placed inside the <channel> and is rarely used. Therefore I didn't followed this.
The individual entries can't have specific <image>-Tags. Images need to be placed into the <description>.
The core RSSFeed class doesn't need to be changed to solve this.
What I did was creating a function called RSSContent() in the page types containing the entries which are used by the feed.
function RSSContent() {
if ($this->Image()->exists())
{
$returnvalue ='<div style="float: left; margin-right: 3px;"><img src="'.$this->Image()->SetWidth(150)->AbsoluteURL.'" alt="'.$this->Image()->Title.'" /></div>'.$this->Content;
}
else
{ $returnvalue=$this->Content; }
return $returnvalue;
}
To be safe a fallback function RSSContent should be placed in the page.php which just returns the usual content.
In the second step the RSS function in the respective holder class needs to be changed:
$rss = new RSSFeed($this->CountryOfTheMonth(), $this->Link(), "Country of the Month", "Each month we present a new country.", "Title", "RSSContent");
$rss->outputToBrowser();
This places image objects in the RSS feeds which are reproduced in the feed reader or in Facebook (when using RSSGraffiti).
(Thanks to Martijn for the help with rendering image objects inside the PHP code)