The short answer is that you can't. But there's a workaround.
Let's say you have a newsletter design as follows, which shows the post
image, followed by its
title and full
*|RSSITEM:IMAGE|* *|RSSITEM:TITLE|* *|RSSITEM:CONTENT_FULL|*
*|RSSITEM:IMAGE|* RSS merge tag will pull in the image and assign to the created
img element the
height corresponding to the original size of the image. Something like this (which is an example extracted from my Mailchimp campaign using the design template above).
<img class="mc-rss-item-img" src="https://nono.ma/img/u/sketch-170202_cambridge-clary-st.jpg" height="1868" width="2500" style="height: 1868;width: 2500;" >
The problem is that we don't want to explicitly specify the
width values of our image, as it's oversized compared to the width of our design, which in my example was
600px plus margins, which was leaving
564px for the width of the image itself.
Here's the workaround: You create the image's
img HTML element yourself, formatting it as you want, and use the
RSSITEM:ENCLOSURE_URL—which according to Mailchimp "Displays the URL for the attached file [of your post]." In my case, the RSS feed is using the
enclosure tag to send the image URL.
// RSS feed item enclosure tag <enclosure url="https://nono.ma/img/u/sketch-170202_cambridge-clary-st.jpg" type="image/jpeg" />
Then I can use that URL in my Mailchimp design as follows (also adding the post's
title and full
content below the
<img src="*|RSSITEM:ENCLOSURE_URL|*" style="max-width: 100%" > *|RSSITEM:TITLE|* *|RSSITEM:CONTENT_FULL|*