The RSS with Images plugin for WordPress allows you to insert images into your blogs RSS feed. You can even add your own image dimensions in the options section. The plugin allows for a minimum of 20px and maximum of 192px for either the height or the width. This means that your minimum image size can be 20×20 pixels and your max image size can be 192×192 pixels.
Before you do ANYTHING check your email template for image size requirements. You don’t want the image to be too small or too big. It may break your layout when the email gets sent out.
Set the character encoding for the XML feed. Chances are you will want to leave this set to the default setting of UTF-8.
Once the width and height are set click Save Settings. Now you can upload a an image in the ‘Featured Image’ section of the post editor. The plugin will hard crop a copy of the image you uploaded and save it in wp-content/uploads/. When it comes time for Mailchimp to dig through your RSS feed, you will have images!
*RSS with Images will not add existing featured images from old posts to your feed. Once the plugin is activated, any new image added to a newly created post will be included in the feed. To remedy this on existing posts with featured images, remove the featured image then re-add, save the post.