Hi Sarah,

Select the ‘page section’ you are trying to edit.

Then in the menu on the right, click ‘Advanced’ > Then Check ‘Stretch background to page edges’ and your done!

Here’s a link to the documentation on it.

http://documentation.unbounce.com/hc/en-us/articles/203510154-Using-Page-Sections

Scroll down until you see the section called ‘Scale to fit container’  and look at the red box

on the right hand side of the second image.

Hope that helps!