If you want to show two columns of Contentbox content constrained to your template width (or even a custom width) you can do this via the Contentbox Settings Popup.
By default setting a Background Colour via the GENERAL PROPERTIES > BACKGROUND COLOUR on a two column Section snippet will set the background colour for the active column.
To set a background colour for the entire Section, click the BACKGROUND OPTIONS section button, then set the colour via the OUTER BACKGROUND COLOUR field.
At this time there is no option to set a Background Image to the entire Section. Background Images can only be applied to the individual Section Column on multi-column layouts.