You can apply a background image to any section using css, just place the id/class of the section in place of the one above. For example #header instead of #container. You can find the class of id for the sections using a web inspection tool like Chrome Dev Tools or Firefox.

If you’ve not used a web inspector before to make css changes, check out https://www.youtube.com/watch?v=yMxIw0KTZ_o which will give you an overview.

http://www.w3schools.com/cssref/pr_background-image.asp