When you insert an image from the media uploader the image code and url is inserted into the HTML are, the image code also contains width and height attribute.
If by any chance you don’t want to display the width & height attribute in the generated image code, this code will be useful. Put the codes in your functions.php
add_filter( 'post_thumbnail_html', 'delete_image_width_height', 20 ); add_filter( 'image_send_to_editor', 'delete_image_width_height', 20 ); function delete_image_width_height( $html ) { $html = preg_replace( '/(width|height)="\d*"\s/', "", $html ); return $html; }