Create Cookie When Link Clicked And Use It As Settings?

I use javascript to create a cookie when you click a link
<a href="#" onclick="document.cookie='settings=blue; expires=31/03/2009;'">Blue</a>
This creates a cookie and I can get it back easily but I want to use it as settings. I have 3 folders of pictures white blue and pink the user will click the link selecting there colour creating a cookie. Now when the page finds the cookie it needs to place the output in to the path.
<img src="pictures/ !!!output!!! /something.png">

So far I have this (just a trial)
<script type="text/javascript" src="cookie.js">
</script>
<script type="text/javascript">
function image(){
settings=getCookie('settings');
if (settings!=null && settings!=""){
document.write('<img src="'+settings+'.png">')
}}document.write(image())
</script>

This works but next to the image it says undefined so how would I get rid of it?