[Octane]

Не нужен здесь никакой z-index.

<style type="text/css">
img {
	border: 0;
	margin: 0;
}
.banners {
	overflow: hidden;
}
.banner:link, .banner:visited, .mask {
	display: block;
	width: 125px;
	height: 125px;
}
.banner:link, .banner:visited {
	position: relative;
	float: left;
	text-decoration: none;
}
.mask {
	position: absolute;
	top: 0;
	left: 0;
	background: url(http://tri4.ax3.net/akado_ural/images/sqban_mask.png) no-repeat;
}
.b2 .mask {
	background-position: 100% 0;
}
</style>
<div class="banners">
	<a class="banner b1" href="#b1">
		<img src="http://tri4.ax3.net/akado_ural/images/sqban3.png" alt="" width="125" height="125" />
		<span class="mask">&nbsp;</span>
	</a>
	<a class="banner b2" href="#b2">
		<img src="http://tri4.ax3.net/akado_ural/images/sqban4.png" alt="" width="125" height="125" />
		<span class="mask">&nbsp;</span>
	</a>
</div>