Wordpress - Stop wordpress from hardcoding img width and height attributes

You can remove the width and height attributes by filtering the output of image_downsize function found at wp-includes/media.php. To do this, you write your own function and execute it via your theme's functions.php file or as a plugin.

Example:

Remove the width and height attributes.

/**
 * This is a modification of image_downsize() function in wp-includes/media.php
 * we will remove all the width and height references, therefore the img tag 
 * will not add width and height attributes to the image sent to the editor.
 * 
 * @param bool false No height and width references.
 * @param int $id Attachment ID for image.
 * @param array|string $size Optional, default is 'medium'. Size of image, either array or string.
 * @return bool|array False on failure, array on success.
 */
function myprefix_image_downsize( $value = false, $id, $size ) {
    if ( !wp_attachment_is_image($id) )
        return false;

    $img_url = wp_get_attachment_url($id);
    $is_intermediate = false;
    $img_url_basename = wp_basename($img_url);

    // try for a new style intermediate size
    if ( $intermediate = image_get_intermediate_size($id, $size) ) {
        $img_url = str_replace($img_url_basename, $intermediate['file'], $img_url);
        $is_intermediate = true;
    }
    elseif ( $size == 'thumbnail' ) {
        // Fall back to the old thumbnail
        if ( ($thumb_file = wp_get_attachment_thumb_file($id)) && $info = getimagesize($thumb_file) ) {
            $img_url = str_replace($img_url_basename, wp_basename($thumb_file), $img_url);
            $is_intermediate = true;
        }
    }

    // We have the actual image size, but might need to further constrain it if content_width is narrower
    if ( $img_url) {
        return array( $img_url, 0, 0, $is_intermediate );
    }
    return false;
}

Attach the new function to the image_downsize hook:

/* Remove the height and width refernces from the image_downsize function.
 * We have added a new param, so the priority is 1, as always, and the new 
 * params are 3.
 */
add_filter( 'image_downsize', 'myprefix_image_downsize', 1, 3 );

Also don't forget to scale the images correctly in your CSS:

/* Image sizes and alignments */
.entry-content img,
.comment-content img,
.widget img {
    max-width: 97.5%; /* Fluid images for posts, comments, and widgets */
}
img[class*="align"],
img[class*="wp-image-"] {
    height: auto; /* Make sure images with WordPress-added height and width attributes are scaled correctly */
}
img.size-full {
    max-width: 97.5%;
    width: auto; /* Prevent stretching of full-size images with height and width attributes in IE8 */
}

Hope this helps you.

Cheers,

You can use the post_thumbnail_html filter to remove the attribute:

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

add_filter( 'post_thumbnail_html', 'remove_img_attr' );

Place this in your functions.php file

You can get featured image URL and add it to your content manually, eg:

<?php 
$image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'thumbnail' ); 

if ($image) : ?>
    <img src="<?php echo $image[0]; ?>" alt="" />
<?php endif; ?>