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.

STEPS:

Download your image

Open Canva

Click NEW

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