In this case, as you want to change the src
of the first value of your element, you have no need to build up a function. You can change this right in the element:
<a href='#' onclick='this.firstChild.src="../template/save.png"')'>
<img src="../template/edit.png" id="edit-save"/>
</a>
You have several ways to do this. You can also create a function to automatize the process:
function changeSrc(p, t) { /* where p: Parent, t: ToSource */
p.firstChild.src = t
}
Then you can:
<a href='#' onclick='changeSrc(this, "../template/save.png");'>
<img src="../template/edit.png" id="edit-save"/>
</a>