With the snippet you provided (and without making assumptions about the parents of the element) you could get a reference to the image with
document.querySelector('img[name="edit-save"]');
and change the src with
document.querySelector('img[name="edit-save"]').src = "..."
so you could achieve the desired effect with
var img = document.querySelector('img[name="edit-save"]');
img.onclick = function() {
this.src = "..." // this is the reference to the image itself
};
otherwise, as other suggested, if you're in control of the code, it's better to assign an id to the image a get a reference with getElementById (since it's the fastest method to retrieve an element)