add featured image to feed (for feeds using excerpts instead of full content). Uses the thumbnail sized image, aligned left, with a margin for the email.

functions.php

<?php

// you can add this to your theme's functions.php file. do not include the opening tag.

add_filter( 'the_excerpt_rss', 'rgc_add_featured_image_to_feed_excerpt', 1000, 1 );
function rgc_add_featured_image_to_feed_excerpt( $content ) {

	if ( has_post_thumbnail( get_the_ID() ) ) {
		$content = get_the_post_thumbnail( get_the_ID(), 'thumbnail', array( 'align' => 'left', 'style' => 'margin-right:20px;' ) ) . $content;
	}
	return $content;
}

mailchimp.txt

*|RSSITEMS:|*
<h2 class="mc-toc-title"><a href="*|RSSITEM:URL|*" target="_blank">*|RSSITEM:TITLE|*</a></h2>
<em>By *|RSSITEM:AUTHOR|*</em><br />
*|RSSITEM:CONTENT|*<br />
*|END:RSSITEMS|*