There are 3 fit modes for banner photos in iNewsletter: Fixed Width, Full Page and Portrait.
Fixed width mode sits the banner photo inside a box (1200 pixels wide) on medium-large screens and full page mode fills the banner image to the width of the screen on all devices.
To change the fit mode
Click to select the banner photo
Click Width, then choose Fixed Width, Full Page, or Portrait (Portrait is not available on the cover 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.