var buttonLocation = 'style/img/';

function doRollOver(theImg, baseName, action) {
    if(action=='up') {
      theImg.src = buttonLocation + baseName + "_up.jpg";
    }
    if(action=='over') {
      theImg.src = buttonLocation + baseName + "_over.jpg";
    }    
    if(action=='down') {
      theImg.src = buttonLocation + baseName + "_down.jpg";
    }
}