Why doesn't this javascript code run ?

I want when the user points to a picture link, the picture will become another one. But the following code will always have erros: document[...] is null or is not an object. What's the problem ?

<script language="JavaScript" type="text/javascript">
<!--
productoff = new Image;
productoff.src = "./img/product.gif";
productover = new Image;
productover.src = "./img/news.gif";


function replaceImg(oldImg,newImg){
document[oldImg].src = eval(newImg + ".src");
}
//-->

</script>


<td><a href="./product.html" onMouseOver = "replaceImg('productoff','productover')" onMouseOut = "replaceImg('productoff','productover')" ><img src = "./img/product.gif" ></a></td>

Give the image tag a name attribute. That's what you're supposed to be passing.

PHP Code:

---

function replaceImg(imgname,newImg){
document[imgname].src = eval(newImg + ".src");
} 


---

When calling it,

HTML Code:

---

<a href="./product.html" onMouseOver="replaceImg('prodimg', 'productover');"><img src="./img/product.gif" name="prodimg"></a>

---

Thanks.