There are two fit modes for banner photos in iNewsletter: full-width and contained.
Contained mode sits the banner photo inside a box (1200 pixels wide) on medium-large screens and full-width mode fills the banner image to the width of the screen on all devices.
You will mostly use contained mode as it is more space efficient, but you may want to draw attention to a page by using full-width mode.
To change the fit mode
On the menu that sits over the top of the banner photo, click the Banner menu
Under Width, choose either Fixed Width or Full Page
If you are working on a small screen you may not see a difference in the fit mode as Full Page is only distinct on medium-large sized screens.