Filters images for RSS feed (Erik Teichmann)

mail.php

/**
 * Filters images for RSS feed--makes thumbs go through large; large images resize down to MailChimp friendly width.
 * Add this to your functions.php file WITHOUT the opening php
 * 
 * Author: Erik Teichmann, minor tweaks by Robin Cornett
 * Author URI: http://www.eriktdesign.com/
 */
 
 // Add filters for RSS
 add_filter('the_excerpt_rss', 'et_change_thumbs', 20); // changes the excerpt for rss
 add_filter('the_content', 'et_change_thumbs', 20); // changes the content
 function et_change_thumbs($content) {  
	 // See if we're dealing with a feed
	 if ( is_feed() ) { // if you're on an RSS feed, do this.
		 // Strip out the parts of img src that lead to thumbnails
		 $content = str_replace("-310x310", "", $content); // looks for thumbail file names to replace with original file name. If your thumbnail settings are different, change this.
		 $content = str_replace("width=\"310\"", "", $content); // default thumbnail width. Change if needed.
		 $content = str_replace("height=\"310\"", "", $content); // default thumbnail height. Change if needed.
		 $content = str_replace("class=\"attachment-thumbnail\"", "style=\"max-width:560px\"", $content); // looks for thumbnail class, resizes to 560px, a MailChimp friendly size (factoring in email on margin)
		 $content = str_replace("width=\"700\"", "style=\"max-width:560px\"", $content); // replaces 700px wide images and makes them smaller. replace 700 with your image size.
	 }
	 // Send the content on its way
	 return $content;  
 }