Falcornus

add_filter( 'post_thumbnail_html', 'remove_thumbnail_dimensions', 10, 3 );

function remove_thumbnail_dimensions( $html, $post_id, $post_image_id ) {
    $html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );
    return $html;
}

Falcornus

<?php if (have_posts() ): query_posts('p=12'); // article ID
  while (have_posts() ): the_post(); ?>
<?php the_title(); ?> <!--Title-->
<?php the_content(); ?> <!--Content-->
<?php the_post_thumbnail(array(500,500)); ?> <!--Image-->

<?php endwhile; endif; wp_reset_query(); ?>