You didn't use the exact code as I posted it, you only used the second part in the code.
This should be in your document:
<a href="" onMouseOver="document.MyImage.src='http://www.phoodie.info/wp-content/uploads/2009/01/number2.jpg';" onMouseOut="document.MyImage.src='http://www.phoodie.info/wp-content/uploads/2009/01/number1.jpg';">This is linked text</a>
<img src="http://www.phoodie.info/wp-content/uploads/2009/01/number1.jpg" border="0" name="MyImage" />
You just put "This is linked text" without putting the <a href (etc)></a> around it, of course it doesn't work without those, as the text is not a link.
Hope to help,