No, that doesn't really tell me anything useful.
What I'm saying in my post is that you need to wrap the target URL you're changing the image's source to in single quotes. You're constructing an eval that looks like this:
document.someName.src = mobilebigimage.jpg
This should be what you're seeing in your alerts. This will not work. You need to be doing this:
document.someName.src = 'mobilebigimage.jpg'
The replacement eval-construction code I posted before is a way to do this.
The reason that nothing like this had to be done before, but has to be done now, is that you're changing methodologies to using an eval, which means that you have to make a string that interprets as valid JavaScript.
But then, the whole methodology you're using isn't really such a good idea. So here's another suggestion for what to do instead of any of this:
<javascript>
<script language="javascript">
function ShowPic(imgname, sImage) {
document[imgname].src = sImage;
}
</script>
If I'm not horribly mistaken, that will work far better and with much less fuss.