/* single thumbnail */
	p.thumb{
		float:left;
		margin:.5em 0;
		margin-right:10px;
		border:1px solid #999;
		padding:2px;
	}
	p.thumb a{
		display:block;
		float:left;
		width:100px;
		height:100px;
		line-height:100px;
		overflow:hidden;
		position:relative;
		z-index:1;
	}
	p.thumb a img{
		float:left;
		position:absolute;
		top:-20px;
		left:-50px;
	}
	/* mouse over */
		p.thumb a:hover{
			overflow:visible;
			z-index:1000;
			border:none;
		}
		p.thumb a:hover img{
			border:1px solid #999;
			background:#fff;
			padding:2px;
		}
	/* // mouse over */
/* // single thumbnail */

<p class="thumb"><a href="http://xyberneticos.com/"><img src="image.jpg" /></a></p>

http://xyberneticos.com/index.php/2008/02/21/cambiar-el-tamano-de-las-miniaturas-usando-la-propiedad-css-overflow/

CSS Cambiar el tamaño de las Miniaturas usando la propiedad Css Overflow

css, image

CSS