if( document.images ) {
   var but01on = new Image(); but01on.src = "img/but01_over.jpg"
   var but01off = new Image(); but01off.src = "img/but01.jpg"
   var but02on = new Image(); but02on.src = "img/but02_over.jpg"
   var but02off = new Image(); but02off.src = "img/but02.jpg"
   var but03on = new Image(); but03on.src = "img/but03_over.jpg"
   var but03off = new Image(); but03off.src = "img/but03.jpg"
   var but04on = new Image(); but04on.src = "img/but04_over.jpg"
   var but04off = new Image(); but04off.src = "img/but04.jpg"
   var but05on = new Image(); but05on.src = "img/but05_over.jpg"
   var but05off = new Image(); but05off.src = "img/but05.jpg"
 }

function imgon( name ) {
  if ( document.images ) {
    document[ name ].src = eval( name + "on.src" )
  }
}

function imgoff( name ) {
  if( document.images ) {
    document[ name ].src = eval( name + "off.src" )
  }
}