https://www.khg-graz.at/swstatic-260930033211/swscripts/std/navtree.js

1/** generic img mouseover function
2* <a href="#" onmouseover="mo(this,'images/mouseover.gif')" onmouseout="mo(this,'')">
3*     <img src="image/standard.gif" alt="alt">
4* </a>
5*/
6function mo(obj, url) {
7	var moimgs;
8	if (this.moimgs == null) this.moimgs = new Array();
9	if (url != '') {
10		var oldurl = obj.getElementsByTagName("img")[0].src;
11		obj.getElementsByTagName("img")[0].src=url;
12		url = obj.getElementsByTagName("img")[0].src; // take what the browser will make of it
13		this.moimgs[url] = oldurl;
14	} else {
15		obj.getElementsByTagName("img")[0].src=this.moimgs[obj.getElementsByTagName("img")[0].src];
16	}
17}