Sometimes background images look great on a computer but not so much on a mobile screen.
You can opt for the website to switch to a plain colour background on mobile devices, while still displaying your background image on larger screens.
To do this, click on Site Settings at the top right of the screen, and click into Background under the 'Look and Feel' section.
Next, click into the Mobile Settings tab.
Here you can select a background colour for mobiles. Click Save once you're done.