var myNum = parseInt(Math.random()*4) + 1;

function HomeImage(){
  document.write("<img src='/Portals/_default/Skins/Rotary/images/homephoto_"+myNum+".jpg'>");
}
function SubImage(){
  document.write("<img src='/Portals/_default/Skins/Rotary/images/subphoto_"+myNum+".jpg'>");
}