Remove P tag from images if aligncenter class is set

it should work like this.

function filter_ptags_on_images($content)
{
    $content = preg_replace('/<p>\s*?((<a.*?>)?<img[^>]+class="[^\"]*aligncenter[^\"]*".*?>(<\/a>)?)?\s*<\/p>/', '$1', $content);
    return $content;
}

it filters if the img tag has the class aligncenter and any class.