Edit in JSFiddle

$(".publicidad").hover(function(){
    $(this).find("img").toggle();
}, function(){
    $(this).find("img").toggle();
});

<div class="publicidad">
    <img src="http://fondosblackberry.com/user-content/uploads/wall/mid/32/Paisajes_manu_peru.JPG" />
    <img src="http://www.fondosblackberry.com/user-content/uploads/wall/mid/78/luna_Moon_behind_tree_paisaje_morado.jpg" />
</div>
  
<div class="publicidad">
    <img src="http://www.fondosblackberry.com/user-content/uploads/wall/mid/54/paisajes_de_peru.jpg" />
    <img src="http://www.fondosblackberry.com/user-content/uploads/wall/mid/6/Autumn_day_river.jpg" />
</div>
  
<div class="publicidad">
    <img src="http://guiaviaje.billetes.com/es/wp-content/uploads/2009/08/01repdominicana_islabacardi_300x250.jpg" />
    <img src="http://www.fondosblackberry.com/user-content/uploads/wall/mid/6/Dona_Ana__Algarve_Portugal1.JPG" />
</div>

.publicidad img:last-child{
    display: none;
}

.publicidad{
    cursor: pointer;
    width: 300px;
}