Random image only to be applied once

[chuckarbs]

I want my website to show a random background image (which I've set up with javascript) but I don't want it to change again while the user is on the site. At the moment it changes every time you click a link, but I'd rather it only change when you hit the Home button.I think this other question is similar, but as I haven't used php much before, I was wondering if there was a javascript alternative? (or just something silly i missing here?)choose random value only onceThanks