if (document.images) {

  BackToCalcOff  = new Image
  BackToCalcOver = new Image

  BackToCalcOff.src  = "images/BackToCalcOff.gif"
  BackToCalcOver.src = "images/BackToCalcOver.gif"

} else {

  BackToCalcOff = ""
  BackToCalcOver = ""
  document.BackToCalc = ""
}