<style type='text/css'>
#boton {
margin: 0 auto;
margin-bottom: 20px;
padding: 0;
text-align: center;
}
#boton a:hover img
{
width: auto;
height: auto;
}
#boton a img
{
width: 250px;
height: 250px;
}
</style>

<div id='boton'>
<a href='http://unviciomas.com' target="_blank">
<img border='0' src='http://unviciomas.com/_ph/2/2/87402351.jpg' title='www.unViciomas.Com'/></a>
</div>