Morgan Kay

If you use a service such as Feedburner or MailChimp to turn your RSS feed into an email subscription, and the images in your posts are more than 600px wide, the formatting of your HTML emails might be messed up. This plugin lets you resize the images in your RSS feeds.
Version 2.0 has been completely re-written from scratch! It is now more reliable, and works with images from external URLs.

RSS Image Resize

Resize images in RSS feeds so they will fit in HTML emails.

Jordy Meow

RSS Featured Image adds the featured image into your RSS feed (in the media:content). It works nicely with Mailchimp (|RSSITEM:IMAGE|). In fact, it basically does the same thing as another (and more famous) plugin but without cluttering your WordPress Admin. This is a simple task, and I believe it should be invisible and run peacefuly in the background, with no impact on the UI and the overall WordPress performance.
<h3>Usage</h3>
By default, the size of the image is “large”. I think it would be a shame to add new options to your WordPress admin only for this, so if you want to change the size, I propose you a filter 🙂 You can include some code in your functions.php to take care of this. This is how you can use it:
function thumbnail_size_for_rss( $default ) { 
 return ‘thumbnail’; 
} 
add_filter( ‘rfi_rss_image_size’, ‘thumbnail_size_for_rss’, 10, 1 );

RSS Featured Image

Add the featured image into your RSS feed (in the media:content). Works nicely with Mailchimp (|RSSITEM:IMAGE|). Light and simple, no options, no clut …

RSS Image Resize vs RSS Featured Image