Force bootstrap responsive image to be square

responsive.php

<div class="col-sm-6"> 
    <a href="#" class="thumbnail">
        <div class="caption">
            title<br/>3 x bekeken
        </div>
        <div class="image">
            <img src="" class="img img-responsive full-width" />
        </div>
    </a>
</div>

.image{
    position:relative;
    overflow:hidden;
    padding-bottom:100%;
}
.image img{
    position:absolute;
}