Looking for Guidance - Structuring the background of a campaign

I am new here and working on our first newsletter template. I would like to structure the background of the message (the sides) to be an image rather than a color. I see where you can choose a section background color, a content background color and turn on a background image, and I have chosen an image, which displays as I’m editing. However, when I send the campaign, it does not show up on the sides of the email. What am I doing wrong? Help please!