function preLoadImages(){
var t='<div style="position:absolute; top:-100px; left:0px; display: none;">';
for(i=0;i<arguments.length;i++)t+='<img src="'+arguments[i]+'" height="1" width="1" alt="" />';
t+='</div>';
document.write(t);
}

preLoadImages('/_images/worldmap_01_europe.gif','/_images/worldmap_02_europe.gif','/_images/worldmap_03_europe.gif','/_images/worldmap_01_asia.gif','/_images/worldmap_02_asia.gif','/_images/worldmap_03_asia.gif','/_images/worldmap_04_asia.gif','/_images/worldmap_02_aus.gif','/_images/worldmap_06_aus.gif','/_images/worldmap_03_africa.gif','/_images/worldmap_04_africa.gif','/_images/worldmap_05_africa.gif');

function swapImage(imgN,imgU){
if(document.images)document.images[imgN].src=imgU;
}