Appearance is everything except content. Below is some information about headers and background and for more go to themes, menus, and widgets.

Headers are the banner photos that go across the top of some themes. This site has a header. To change the header, go to Appearance- header. Upload/choose a photo from your media library. You’ll likely be asked to crop the photo; it’s advisable to keep the crop ratio already set for you or to alter it only slightly.

Background varies from theme to theme. Some themes cater more towards a photo as a background, and others just a constant color. Experiment around with each and see what you like.
Check out some free background images on this website, or you can preform a Google search (answers about finding and citing pictures here).