-First log into your twitter account. Then click on settings (look at the example below).

-Then you should come to a page like the one below (look at the example below). Click on Design (look at the example below).

-Next you should come to a page with background themes then below them you will see something that says change background image (look at the example below).

-After you have click on change background image you will come to a page were you can upload a background (look at the example below). Click on browse and find the background you saved that you want to use.
then make sure tile background is checked (look at the example below). Then click the button that says save changes (look at the example below).