Delay loading of background image

I have set a background image for a div to show only when the ad in the div is blocked. Upon page loading though, the background image appears first, then the ad appears.

I have tried the following functions, but they give me syntax errors. I would appreciate having either function cleaned up, or I would certainly try an alternative…thanks.

function loadBG(){
    document.getElementById("id").style.backgroundImage="/images/image.png";
}
window.onload=loadBG();

function show() {
    AB = document.getElementById('id');
    AB.style.display = 'inline';
}
setTimeout("show()", 5000);

-Scott