function rollover() {
  
  var randomnumber1=Math.floor(Math.random( )*2) + 1;
  var randomnumber2=Math.floor(Math.random( ) * 12) + 1;
  
  var imgsource = 'jump' + randomnumber1;
  var imgname = 'roll' + randomnumber2 + '.jpg';
  
  var thisimg = document.getElementById(imgsource);
  
  // and add "_on" to its src
  //thisimg.src = thisimg.src.replace(/(\.[a-z0-9]+)$/i,'_on$1');
	thisimg.src = 'img/' + imgname;
}