<style type="text/css">
.a img {
-moz-transition: all 1s ease-in-out;
-webkit-transition: all 1s ease-in-out;
-o-transition: all 1s ease-in-out;
transition: all 1s ease-in-out;
position:absolute; left: 0; top: 0;
}
.a img.b1 {opacity:0;filter:alpha(opacity=0);}
.a:hover img.b1 {opacity:1;filter:alpha(opacity=100);}
.a:hover img.b2, .demo img.b2:hover {opacity:0;filter:alpha(opacity=0);}
</style>

<div class="a">
<img src="http://webo4ka.ru/webo4kaRu/img/logo/logoWebo4kaRu.png" class="b1" />
<img src="http://webo4ka.3dn.ru/_ld/36/s21264871.jpg" class="b2" />
</div>