My last childnode is an img and is undefined according to firebug.
Can I do this?
this.parentNode.parentNode.parentNode.parentNode.previousSibling.childNodes.childNodes.src = "/assets/img/icons/process.png";
childNodes returns an array-like collection, so you'll need to pick one of those items if you want to get its own childnodes.
There's little else that can be advised without knowing much of the HTML that's involved too.