Header and site backgrounds can be added by the site administrator in site customisation. Go to back ground images section and select a background for the site once uploaded you can then customise how the image appears position and size etc.

 

You can also add a header image via the same process that will appear behind your title, the header image should be no larger than 1280x170, it will cut off edges if image exceeds this size. Once satisfied select save & publish.