Background Image – Multiple Resolutions

That’s a wrong way to put an advertisement banner.

The reason it shows up is because the device you use to view the site is way bigger than the content width, so the background shows up on both sides.

Suggestion is to find another way, the proper way, to put up the banner, like sidebar widget, or create a child theme and put in the banner at the top most, or before posts, and make sure the image has responsive CSS class, also the image itself must look good when scaled down to 320px.