<style type="text/css"> /*Фоновый рисунок*/
html, body {
background: transparent none !important;
}
</style>
<img id="Day" src="http://s019.radikal.ru/i619/1204/05/420ff6c4bc24.gif" style="display:none;position: fixed; top: 0; left: 0; z-index: -1; width: 100%; height: 100% ; margin: 0 0 0 0;"/>
<img id="Night" src="http://s44.radikal.ru/i105/1204/9a/6b06fdf59cb4.gif" style="display:none;position: fixed; top: 0; left: 0; z-index: -1; width: 100%; height: 100% ; margin: 0 0 0 0;"/>
<script language="JavaScript">
theTime = new Date();
theHour = theTime.getHours();
if(theHour>7&&theHour<20){$("#Day").show();}
else{$("#Night").show();}
</script>