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<?php
//Function to link images in GALLERIES to large versions instead of originals. From here http://oikos.org.uk/2011/09/tech-notes-using-resized-images-in-wordpress-galleries-and-lightboxes/, used on Moscovita.it
function oikos_get_attachment_link_filter( $content, $post_id, $size, $permalink ) {
 
    // Only do this if we're getting the file URL
    if (! $permalink) {
        // This returns an array of (url, width, height)
        $image = wp_get_attachment_image_src( $post_id, 'large' );
        $new_content = preg_replace('/href=\'(.*)\'/', 'href=\'' . $image[0] . '\'', $content );
        return $new_content;
    }
}
add_filter('wp_get_attachment_link', 'oikos_get_attachment_link_filter', 10, 4);
?>