RSS with thumbnails

How can i add a thumbnail image into the RSS feed from news articles? I am using contao core RSS system and also have tried [summarizeFeeds] extension.
Thanks

For future reference, here is my solution:
I edited system/module/news/News.php around line 133 and changed
$objItem->description =(($arrArchive['source'] == 'source_text') ? $objArticle->text : $objArticle->teaser);
to:

Code:

---

$TempImage=(($objArticle->source == 'external') ? '' : $strLink).$this->getImage($objArticle->singleSRC, 150,null);
                        $objItem->title =$objArticle->headline;
                        $objItem->description = '[img]'.$TempImage.[/img]".(($arrArchive['source'] == 'source_text') ? $objArticle->text : $objArticle->teaser);

---

As you noticed i used an extra variable $TempImage
If anybody finds a better solution, feel free to post it.