Choosing a background for all text areas
You can find our stock images on the "Contents" tab. Click on the backgrounds button, which is the second furthest right button on the "Contents" section.

Switch to background mode by clicking the icon. You can find this on the bottom left corner.

Add a background to the middle area. This will add a background to the main container that contains ALL of the layout blocks within your design.

Here is what the template will look like once you drag and drop an image:

If you change to the layout view by clicking on the bottom left icon, then here is what it will look like:

Notice that the layout handles are back. As you can see, the background only applies to the middle portion of the email. The background surrounds the layout blocks.
Choosing an email background