a.portfolio
{
	display: block;
	width: 350px;
	height: 131px;
	background: url("images/portfolio.png") 0 0 no-repeat;
	position: absolute;
	top: 100px;
	left: 200px;
}
a:hover.portfolio
{
	background-position: -350px 0;
}

<a href="external2.htm?keepThis=true&TB_iframe=true&height=600&width=800" title="blank" class="thickbox"><div id="portfolio">asdf</div></a>