.ex1 img{
border: 5px solid #ccc;
float: left;
margin: 15px;
-webkit-transition: margin 0.5s ease-out;
-moz-transition: margin 0.5s ease-out;
-o-transition: margin 0.5s ease-out;
}
.ex1 img:hover {
margin-top: 2px;
}
<div class="ex1"><img src="imagenes/ejemplo.png" /></div>