Mailchimp Merge Tag use Free vs Premium
There are two ways our plugin works within Mailchimp, the free way, and the Premium way.
The free way, it prepends the <description> part of the RSS feed with the image URL, and so in Mailchimp using the RSSITEM:CONTENT or RSSITEM:CONTENT_FULL merge tag will show the image successfully, with two caveats: one, if your website is not on https, Mailchimp's Preview Mode has an issue with the images, but a test or real email send should work fine... and two, you may need some custom CSS to style/size it:
The premium way (and we have a 14 day free trial), when you check the Media Tag option, it creates and includes the Media:Content block, and then in Mailchimp you can use the RSSITEM:IMAGE merge tag, and place the image wherever you like, create custom designs, etc. Those same two caveats apply, non-https websites may not show the images in Preview Mode but should work when sent, and custom CSS is even more desirable, which we can help with, and which is why it's a Premium option.
Here's more info on Mailchimp custom CSS:
Often Mailchimp requires custom CSS using it's own CSS classes to affect the images it's adding to the template. You would paste it into the RSS merge tag body Text Block, click on it's Code <> icon, and paste it at the very top of the HTML code.
For example, this CSS would make the images full-width above the text:
This CSS would make it 250px wide, left aligned, with a 15px margin between the image and text: