var jobsArray = new Array(5);

jobsArray[0]='../images2/black-careers1.jpg';
jobsArray[1]='../images2/black-careers2.jpg';
jobsArray[2]='../images2/black-careers3.jpg';
jobsArray[3]='../images2/black-careers4.jpg';
jobsArray[4]='../images2/black-careers5.jpg';

var p = jobsArray.length;
var preLoad = new Array();
for (i = 0; i < p; i++){
   if (document.images){
   preLoad[i] = new Image();
   preLoad[i].src = jobsArray[i];
   }
}

var randomImage = Math.round(Math.random()*(p-1));


function swap() {
    document['imgMain'].src = jobsArray[randomImage];
}