Thanks for asking.
Actually the image is using all the available space on the canvas. If you would like the image to be fullscreen, you will have to change the layout either of the section or the page depending on the requirement. This is the current layout.
You might have to change to this layout.
However, please note that it will break the flow of layout and things won’t look good. My suggestion would be to use the current layout and play with the style to see which layout meets your requirement.
To learn more about X Theme and Cornerstone, please take a look at KB resources.