See if this meets your needs...
Code:
<!DOCTYPE HTML>
<html>
<head>
<title> Logo Script </title>
<script type="text/javascript">
var logos = [
'http://www.nova.edu/hpd/otm/pics/4fun/11.jpg',
'http://www.nova.edu/hpd/otm/pics/4fun/21.jpg'
];
onload = function () {
var now = new Date();
var DOW = now.getDay();
var HH = now.getHours();
if ( ((HH >= 8) && (HH < 18)) && ((DOW >= 1) && (DOW <= 6)) ) {
document.getElementById('logo').src = logos[0];
} else {
document.getElementById('logo').src = logos[1];
}
}
</script>
</head>
<body>
<img id="logo" src="" alt="">
</body>
</html>