When you upload an image through the WordPress media uploader and then insert it into the editor, it comes with width and height attributes. These are normally desirable, as it assists the browser in making the appropriate room for the image during layout. But if you want to remove the insert action from adding these attributes, you can add this code to you functions.php file or a functionality plugin of your own making:

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;
}