var myImg= new Array(3)
  myImg[0]= "1";
  myImg[1]= "2";
  myImg[2]= "3";
  myImg[3]= "4";

myImgSrc = "";

myImgEnd = ".jpg"

var i = 0;

function loadImg(){
  document.imgSrc.src = myImgSrc + myImg[i] + myImgEnd;
}

function prev(){
  if(i<1){
    var l = i
  } else {
    var l = i-=1;
  }
  document.imgSrc.src = myImgSrc + myImg[l] + myImgEnd;
}

function next(){
  if(i>2){
    var l = i
  } else {
    var l = i+=1;
  }
  document.imgSrc.src = myImgSrc + myImg[l] + myImgEnd;
}

window.onload=loadImg;