If you would like to add a background picture to your profile, click on the profile icon in the upper right-hand corner of the dashboard to go to your profile page. Then, click on the camera icon in the lower right-hand corner of the banner at the top of your profile. A menu will display where you can either pick an image from the media library or upload your own image.
Once you have chosen an image, you can adjust it by using the line tool (immediately to the left of the "Save" and "Cancel" buttons) to drag the image up or down. Once you're satisfied with the image, click "Save." Then, toggle the eye-shaped icon in the upper right-hand corner of your profile page to turn off edit mode and save all changes to your profile.