<Script Language="JavaScript">   
  var imagesarr = new Array(   
  "book1.jpg",   
  "book2.jpg",   
  "book3.jpg");   
  var commentsarr = new Array(   
  "Alt - 1",   
  "Alt - 2",   
  "Alt - 3");   
  </Script>

<Script Language="JavaScript">   
  var los = Math.floor(Math.random() * imagesarr.length)   
  document.write ("<Img Src='"+imagesarr[los]+"' Alt='"+commentsarr[los]+"'>");   
  </Script>