Replacing the image URL field with a drop down

Firstly, I have made it into a drop down list of images, that's no problem. You can insert them fine, also.

The only area I have having difficulty is making the image appear in the preview box. The code I currently have is:

id:'textUrl',
type:'select',
label:'',
items:[
    ['Google','http://www.google.co.uk/intl/en_uk/images/logo.gif'],
    ['Yahoo','http://l.yimg.com/a/i/ww/met/yahoo_logo_us_061509.png']
],
onChange:function(){
    var t=CKEDITOR.document.getById('previewImage');
    var q=this.getDialog();
    r=this.getValue();
    if(r.length>0){
        q=this.getDialog();
        var s=q.originalElement;
        s.setCustomData('isReady','false');
        var t=CKEDITOR.document.getById('ImagePreviewLoader');
        if(t){
            t.setStyle('display','');
        }
        s.on('load',o,q);
        s.on('error',p,q);
        s.on('abort',p,q);
        s.setAttribute('src',r);
        q.preview.setAttribute('src',r);
        h(q);
    }
},

The default code in the onChange function causes Javascript errors - I was wondering if anyone knew why, and how to fix it?

Thanks!