Mailchimp has a cute feature where you can crop your image into a circle, directly in the builder. So why isn't it working when you use it? If you're here, your image is probably turning into an oval. It's because your original image is not a perfect square.
Download your image
Choose "Instagram Post"
Drag in your image
Center it to fit the square shape
Download as a PNG or JPEG from Canva
Upload to Mailchimp
Click STYLE and set the "Rounded Corners" adjustor all the way to the right