<img id="img" src="cat.jpg" />​

$('#img').toggle(
    function(){
        $(this).attr('src', 'dog.jpg');
    },
    function(){
        $(this).attr('src', 'cat.jpg');
    }
);​

var images = ["hackanm.gif", "compman.gif"];
var index = 0;

function changeSrc() {
  document.getElementById("myImage").src = images[index];
  index = (index + 1) % images.length;
}