Support for ActiveCampaign?

Yes, our plugin adds the image HTML to the front of the Description (Long) RSS item, which you can see in this image of theirs:
https://media.screensteps.com/image_assets/assets/001/840/477/original/0354cad3-c070-4bac-a0ab-c4ca11f52a1a.png

and as shown in their Images in RSS help article here:
https://help.activecampaign.com/hc/en-us/articles/115001387690-How-do-I-show-images-in-my-RSS-email-

Also, our premium version (perhaps try the 14 day free trial) supports Media and Enclosure tags as well, which may enable more advanced design in the email template system, per the Content URL and Enclosure URL items they mention in that same article.