.moose img{width:100%;height:auto}

.moose{
width:px;
height:auto;
float:;
margin:px;
}
/* */

<!-- -->
<dIv class="moose">
</dIv>

WordPress Thumbnail (put add_theme_support('post-thumbnails'); in functions.php)

<?php if ( has_post_thumbnail() ) { ?>
<div class="moose">
<?php the_post_thumbnail( '100%' ); ?>
</div>
<?php } ?>