hrecipe microformat tag to “featured image”

Use the filter post_thumbnail_html. The callback takes 5 parameters, the first ($html) is the one that’s returned and that we modify, and the others can be helpful to build the modification.

add_filter( 'post_thumbnail_html', 'add_tag_to_featured_wpse_95469', 10, 5 );

function add_tag_to_featured_wpse_95469( $html, $post_id, $post_thumbnail_id, $size, $attr )
{
    $modify = str_replace( '<img', '<img itemprop="photo"', $html );
    return $modify;
}

_{Relevant Q&A: Where to put my code: plugin or functions.php?}