Changing Background

When I initially set up my website I had a background set up that didn’t scroll. I had edited some file to ensure that this happened. However, when I went to change the background (seasonal) it had no effect on the actual background ??

Stupidly I then deleted the old background out of the media file and now I’m really stumped. Absolute Newbie here so would appreciate help on putting in a new background.

Web Address: https://www.groundtechwales.co.uk