In your CommBox signage portal, select 'Playlists' on the left side of the screen and open the playlist with the images you would like to edit. In the fit column, you will have the option to make the content Fit, Fill or Stretch. 

 The 'Fit' option will fit the image to the screen with black strips on either side.  The 'fill' option will maintain the aspect ratio of the image while zooming it in slightly. The other option is 'Stretch' which will stretch the image. The stretch set can have some abnormal outcomes if the content is not the right resolution.