Changing a non-background image on hover with CSS

gistfile1.html

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gistfile2.css

a {
	position:relative;
	display: block;
	height: 30px;
	width: 245px;
	overflow: hidden;

}
a img {
	position:absolute;
	left:0px;
	top:0px;
}

a:hover img {
	top: -30px;
}