celý to je v <body>..
Kód: Vybrat vše
<div id="Pozadi" style="background-image: url('img/va.jpg')" class="main">
<script>
function ZmenaPozadi() {
document.getElementById("Pozadi").style.backgroundImage = "url('img/va1.jpg')";
}
</script>
<button type="button" onclick="ZmenaPozadi()">Zmen pozadi</button>