Go to Layout>Edit HTML>and find the coding that says:



<style>body {background-image: url("URL HERE"); background-position: center; background-repeat:repeat; background-attachment: fixed; }</style>
(it's only about 5 lines down from the top)
then insert this coding:

<style>body {background-image: url("URL HERE"); background-position: center; background-repeat:repeat; background-attachment: fixed; }</st

directly below it.
Click save and you're done!