var imageNum = 0;
var imageArray = Array ("1.png", "2.png", "3.jpg");

function next () {
   imageNum++;
   if  (imageNum >2 ) { //if imageNum is greater than four, then reset it to 0
       imageNum =0;  
   } 
   document.getElementById("logo").src = "images/logo/"+imageArray[imageNum];
   
}