function imgSwap(myObj){
  mc=myObj.childNodes[0]; mcs=mc.src;
  if( mcs.match('-over') ) mcs = mcs.replace( /-over.gif/, '.gif' );
    else mcs = mcs.replace( /.gif/, '-over.gif' );
  mc.src=mcs;
}//imgSwap

window.onload = function(){
  for(var i=1;i<=7;i++){
    document.getElementById('btn_'+i).onmouseover = function(){ imgSwap(this); };
    document.getElementById('btn_'+i).onmouseout = function(){ imgSwap(this); };
  }
}