How to add other background to theme

Hello 🙂
I faced some issue while trying to add custom background ( http://s19.postimage.org/thw4dvag1/trans.png ) to that theme ( http://www.web2feel.com/infrared/ ) . I tried to replace the “menu” and “trans” but my background did not changed. I really want to change my nasty background to that mentioned above. Any help or advice how to change it will make me the happiest man on the world.
website: ukhardware.tk

Regards George.