RSS feeds – adding images and links to summarized feed?

The documentation on this, plus any sort of instruction, is severely lacking online.

I discovered that in the main directory, wp-includes/feed.php appears to be the right feed I need to edit to modify my template.

How do I truncate a post’s text while keeping links and images intact? And what do I need to do (either in the feed or in the post template) to add a thumbnail image, but not the rest of the images in a post?