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<?php
//filter <p> on images
function filter_ptags_on_images($content){
   return preg_replace('/<p>\s*(<a .*>)?\s*(<img .* \/>)\s*(<\/a>)?\s*<\/p>/iU', '\1\2\3', $content);
}

add_filter('the_content', 'filter_ptags_on_images');