When/how are the K2FeedImage & K2FeedIntroText classes used for?
Where is the Channel <description> coming from?

Hello Joe,

The feed is generated in the /components/com_k2/views/itemlist/view.feed.php

The description of the item is the <description> cell of the feed. It is generated in the item model.

The classes are defined inside the models as well.
/components/com_k2/models/item.php and components/com_k2/itemlist.php

I would not hack these files because since they cannot be overridden, you will lose your changes when you update.