If you would like the background image to be something other than the default background set by TeamLinkt.
*Not available on all templates
Classic Website Editor:
Go to the Website tab and click Manage
Edit your Home page (or whatever you renamed it to) and upload a new background image under the Page Settings section
Press 'Save'
Website Builder:
Go to the Website tab and click Builder
Select your Home page (or whatever you called it) from the Pages list at the top
Click the background image in the top header
Upload a new image
Once you press Save it will be immediately applied, you will not need to press Save Draft or Publish
NOTE: an image that is 1920 x 520 pixels in size in recommended