Slightly smarter filtering to remove hard-coded width and height attributes from *all* images in WordPress (post thumbnails, images inserted into posts, and gravatars). Handy for responsive designs. Add the code below to the functions.php file in your theme's folder (/wp-content/themes/theme-name/ ). Remember to rename the function as needed to …

gistfile1.php

/**
* Filter out hard-coded width, height attributes on all images in WordPress. 
* https://gist.github.com/4557917
*
* This version applies the function as a filter to the_content rather than send_to_editor. 
* Changes made by filtering send_to_editor will be lost if you update the image or associated post 
* and you will slowly lose your grip on sanity if you don't know to keep an eye out for it. 
* the_content applies to the content of a post after it is retrieved from the database and is "theme-safe". 
* (i.e., Your changes will not be stored permanently or impact the HTML output in other themes.)
*
* Also, the regex has been updated to catch both double and single quotes, since the output of 
* get_avatar is inconsistent with other WP image functions and uses single quotes for attributes. 
* [insert hate-stare here]
*
*/
function mytheme_remove_img_dimensions($html) {
  $html = preg_replace('/(width|height)=["\']\d*["\']\s?/', "", $html);
    return $html;
}
add_filter('post_thumbnail_html', 'mytheme_remove_img_dimensions', 10);
add_filter('the_content', 'mytheme_remove_img_dimensions', 10);
add_filter('get_avatar','mytheme_remove_img_dimensions', 10);

Yeah, this doesn't play nice with the FitVids.js convention

@jeremyclarke Yes, this should remove all width and height attributes from all elements (which is what I was aiming for in this case). Can look into modifying the regex to filter only and/or create a conditional, like @laacz has done.

@laacz Very nice, thanks!

@laacz actually I believe that it will work better using "preg_match_all" instead of "preg_match" to get all the occurences. Otherwise you will actually get only the first image of the content. That's at least what I got in my tests. I now use the function like this :

function remove_thumbnail_dimensions($html) {
    // Loop through all <img> tags
    if (preg_match_all('/<img[^>]+>/ims', $html, $matches)) {
        foreach ($matches as $match) {
            // Replace all occurences of width/height
            $clean = preg_replace('/(width|height)=["\'\d%\s]+/ims', "", $match);
            // Replace with result within html
            $html = str_replace($match, $clean, $html);
            error_log($match);
        }
    }
    return $html;
}

@stuntbox is there a reason you've added the hook at the_content, rather than 'image_send_to_editor'? Is it purely for backward compatibility with existing posts?