functions.php

<?php
/* Remove the width and height attributes from wp image uploader.
   http://blog.wpjam.com/m/how-to-remove-the-width-and-height-attributes-from-wp-image-uploader/
   ------------------------------------------------------------------------------------------ */

add_filter( 'post_thumbnail_html', 'remove_width_attribute', 10 );

add_filter( 'image_send_to_editor', 'remove_width_attribute', 10 );

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

?>