function preLoadImages(){
var t='<layer top="-100" left="0" visibility="hide"><div style="position:absolute; top:-100px; left:0px; visibility:hidden">';
for(i=0;i<arguments.length;i++)t+='<img src="'+arguments[i]+'" height="10" width="10">';
t+='</div></layer>';
document.write(t);
}

preLoadImages('images/rooms/gf_plan_drawing.gif','images/rooms/gf_plan_dining.gif','images/rooms/gf_plan_library.gif','images/rooms/gf_plan_conservatory.gif'); 

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