Thanks for following up with that information! It was very helpful for visualizing and troubleshooting the issue here.
I took a look at that campaign, and I see that those images in the columns are not rendering responsively when swapped out in the campaign builder. I checked out your code for that template as well, and it looks like it’s set up to have fluid images there. The code also seems to be set up to shrink down high definition images when they are brought into that campaign.
In this case, it looks like when your template is being tested, edited, and a campaign is drafted with that template, everything works as expected. However, when that image is swapped out in the campaign builder directly, the image that is brought in no longer has the width designated for high-res images, and stops rendering in the appropriate size. To provide some background here, when an image is replaced in the campaign builder the image properties such as the width set in the tag are removed as they were specific to that image.
One thing that can be done to work around this is to have those images be changed in the template’s coding directly rather than in the campaign builder. This should allow those images to keep that designated width and keep them from staying at that larger size. Another alternative to consider is to host those images in a URL outside of MailChimp, and upload the image to that URL when needed. If the URL remains unchanged, it should apply those size settings to that URL even if the image is swapped out server-side. This option may be a bit more client-friendly than editing the code itself.
That should get things moving in the right direction! Please feel free to contact us if you have any additional questions.