Banner image doesn't scale well when expanding size of web browser window

[finite9]

When you look at a Facebook page or YouTube channel page, the banner image on the page is displayed in full no matter what size of window you have, whether it be mobile screen or desktop-browser. If you resize the desktop browser window to a very narrow view, then drag the window size out to expand it's width, the banner image is scaled dynamically. The full image is always displayed no matter the width.

In Pelican, with pelican-bootstrap3 theme, the image is only full when viewing on mobile platforms or very narrow browser windows. When I drag the browser window out to increase it's width, the image zooms in to display only a small portion of my banner.

Is this due to the way the code in the templates work? It doesn't seem to matter which banner image size proportions I choose.