Edit button

On an iPhone when you open a new tab scroll down all the way to the bottom there should be a button that says edit click on it. The bottom toggle says background image enable that and you will be able to change the background image. If you are trying to do this on a Mac when you open a new tab you should see the button that looks like three sliders in the bottom right corner click on it and enable the same toggle and you will be able to choose your background image.

Open safari and make sure that you're opening a normal tab (not a private one ) scroll down 'till you find the edit button , tap it and you'll be able to customize you browser's background