[Zaja]

Try following:

1. Head:

<script type="text/javascript">
//<![CDATA[

{literal}
var w=screen.width;
var h=screen.height;

function resolution() {
if((w==800)&&(h==600)) {
document.getElementById('point').src='skin1/images/800_600_bg.jpg';
}
else if((w==1024)&&(h==768)) {
document.getElementById('point').src='skin1/images/1024_768_bg.jpg';
}
else if((w==1152)&&(h==864)) {
document.getElementById('point').src='skin1/images/1152_864_bg.jpg';
}
else if((w==1280)&&(h==960)) {
document.getElementById('point').src='skin1/images/1280_960_bg.jpg';
}
else if((w==1280)&&(h==1024)) {
document.getElementById('point').src='skin1/images/1280_1024_bg.jpg';
}
}
onload=resolution;
{/literal}

//]]>
</script>

2. Body:

<div>[img]skin1/images/main_bg.jpg[/img]</div>