Your Google Plus background cover gives you an opportunity to promote your business or any other information you want people to notice about your profile. You have a relatively big space to use on whatever you choose. Keep in mind that the image is 1902pxl x 1070 pxl so if you want to use a picture to cover most of the background it must be a large picture of good quality (enlarging a picture to fit will produce a poor quality image).
Your cover is very easy to change you simply login to your Google Plus account, go to your profile. You will see on the main picture at the bottom ‘Skift forsidebillede/Change cover, click and select Upload. Simply upload the picture from your files, enlarge it to fill the full space by dragging the corners and then ‘Vælg et forsidefoto/Choose cover photo’ at the bottom left of the page.