image_ptag_remover.php

<?php
/*
Plugin Name: Image P tag remover
Description: Plugin to remove p tags from around images and iframes in content outputting, after WP autop filter has added them. (oh the irony)
Version: 1.1
Author: Fublo Ltd
Author URI: http://blog.fublo.net/2011/05/wordpress-p-tag-removal/
*/

function filter_ptags_on_images($content)
{
    // do a regular expression replace...
    // find all p tags that have just
    // <p>maybe some white space<img all stuff up to /> then maybe whitespace </p>
    // replace it with just the image tag...
    $content = preg_replace('/<p>\s*(<a .*>)?\s*(<img .* \/>)\s*(<\/a>)?\s*<\/p>/iU', '\1\2\3', $content);
    // now pass that through and do the same for iframes...
    return preg_replace('/<p>\s*(<iframe .*>*.<\/iframe>)\s*<\/p>/iU', '\1', $content);
}

// we want it to be run after the autop stuff... 10 is default.
add_filter('the_content', 'filter_ptags_on_images');