In the screenshot below you can see that the image is position a bit low in the window when centered.
To change the background position, click the Contentbox Settings icon, either top-left, or bottom right of the section.

With the Section settings popup showing, choose the BACKGROUND SIZE / POSITION section and then the BACKGROUND POS setting.

Changing the background pos to CENTER BOTTOM (Down Arrow), retains the horizontal centering but vertically locks the image to the bottom of the sections rather than the center.

When viewed on the site the background is now locked to the bottom on the site.