Text Size Selector

I wanted to add a text size selector thingy to my church's website. So, I made a script and added some buttons, but I keep getting errors. I'm pretty much a n00b at JS, so... Sorry if this is a really stupid question.

Anyway, here's the function:

Code:
function setFontSize(fontsize) {

/*
* Part I: Change The Size
*/
var class=(fontsize=="large") ? "contentLargeFont" : "content";
getElementById('contentDiv').className=class;

/*
* Part II: Set The Cookie
*/
var expdate=new Date();
expdate.setTime((expdate.getTime())+(60*60*24*365));
document.cookie="fontsize="+escape(fontsize)+";expires="+expdate;

}
And then here's what I use to call the function.


HTML Code:
<img src="/tcc/img/smallfont.gif" title="Normal Size" onClick="setFontSize('small')">
<img src="/tcc/img/largefont.gif" title="Large Size" onClick="setFontSize('large')">