Overide Gallery Default Link to Settings

Unfortunately there is no legal way to control it. But there is a dirty way to do it… If you select this route, then you will need to :

• clone the standard gallery_shortcode function
• add a default value for $attr['link'] option
• hook your cloned function into post_gallery filter

The final result will look like this:

add_filter( 'post_gallery', 'wpse8170_gallery_shortcode', 10, 2 );
function wpse8170_gallery_shortcode( $output, $attr ) {
    $post = get_post();

    static $instance = 0;
    $instance++;

    // override default link settings
    if ( empty(  $attr['link'] ) ) {
        $attr['link'] = 'none'; // set your default value here
    }

    if ( !empty( $attr['ids'] ) ) {
        // 'ids' is explicitly ordered, unless you specify otherwise.
        if ( empty( $attr['orderby'] ) )
            $attr['orderby'] = 'post__in';
        $attr['include'] = $attr['ids'];
    }

    // We're trusting author input, so let's at least make sure it looks like a valid orderby statement
    if ( isset( $attr['orderby'] ) ) {
        $attr['orderby'] = sanitize_sql_orderby( $attr['orderby'] );
        if ( !$attr['orderby'] )
            unset( $attr['orderby'] );
    }

    extract(shortcode_atts(array(
        'order'      => 'ASC',
        'orderby'    => 'menu_order ID',
        'id'         => $post ? $post->ID : 0,
        'itemtag'    => 'dl',
        'icontag'    => 'dt',
        'captiontag' => 'dd',
        'columns'    => 3,
        'size'       => 'thumbnail',
        'include'    => '',
        'exclude'    => ''
    ), $attr, 'gallery'));

    $id = intval($id);
    if ( 'RAND' == $order )
        $orderby = 'none';

    if ( !empty($include) ) {
        $_attachments = get_posts( array('include' => $include, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );

        $attachments = array();
        foreach ( $_attachments as $key => $val ) {
            $attachments[$val->ID] = $_attachments[$key];
        }
    } elseif ( !empty($exclude) ) {
        $attachments = get_children( array('post_parent' => $id, 'exclude' => $exclude, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );
    } else {
        $attachments = get_children( array('post_parent' => $id, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );
    }

    if ( empty($attachments) )
        return '';

    if ( is_feed() ) {
        $output = "\n";
        foreach ( $attachments as $att_id => $attachment )
            $output .= wp_get_attachment_link($att_id, $size, true) . "\n";
        return $output;
    }

    $itemtag = tag_escape($itemtag);
    $captiontag = tag_escape($captiontag);
    $icontag = tag_escape($icontag);
    $valid_tags = wp_kses_allowed_html( 'post' );
    if ( ! isset( $valid_tags[ $itemtag ] ) )
        $itemtag = 'dl';
    if ( ! isset( $valid_tags[ $captiontag ] ) )
        $captiontag = 'dd';
    if ( ! isset( $valid_tags[ $icontag ] ) )
        $icontag = 'dt';

    $columns = intval($columns);
    $itemwidth = $columns > 0 ? floor(100/$columns) : 100;
    $float = is_rtl() ? 'right' : 'left';

    $selector = "gallery-{$instance}";

    $gallery_style = $gallery_div = '';
    if ( apply_filters( 'use_default_gallery_style', true ) )
        $gallery_style = "
        <style type="text/css">
            #{$selector} {
                margin: auto;
            }
            #{$selector} .gallery-item {
                float: {$float};
                margin-top: 10px;
                text-align: center;
                width: {$itemwidth}%;
            }
            #{$selector} img {
                border: 2px solid #cfcfcf;
            }
            #{$selector} .gallery-caption {
                margin-left: 0;
            }
            /* see gallery_shortcode() in wp-includes/media.php */
        </style>";
    $size_class = sanitize_html_class( $size );
    $gallery_div = "<div id='$selector' class="gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}">";
    $output = apply_filters( 'gallery_style', $gallery_style . "\n\t\t" . $gallery_div );

    $i = 0;
    foreach ( $attachments as $id => $attachment ) {
        if ( ! empty( $attr['link'] ) && 'file' === $attr['link'] )
            $image_output = wp_get_attachment_link( $id, $size, false, false );
        elseif ( ! empty( $attr['link'] ) && 'none' === $attr['link'] )
            $image_output = wp_get_attachment_image( $id, $size, false );
        else
            $image_output = wp_get_attachment_link( $id, $size, true, false );

        $image_meta  = wp_get_attachment_metadata( $id );

        $orientation = '';
        if ( isset( $image_meta['height'], $image_meta['width'] ) )
            $orientation = ( $image_meta['height'] > $image_meta['width'] ) ? 'portrait' : 'landscape';

        $output .= "<{$itemtag} class="gallery-item">";
        $output .= "
            <{$icontag} class="gallery-icon {$orientation}">
                $image_output
            </{$icontag}>";
        if ( $captiontag && trim($attachment->post_excerpt) ) {
            $output .= "
                <{$captiontag} class="wp-caption-text gallery-caption">
                " . wptexturize($attachment->post_excerpt) . "
                </{$captiontag}>";
        }
        $output .= "</{$itemtag}>";
        if ( $columns > 0 && ++$i % $columns == 0 )
            $output .= '<br style="clear: both" />';
    }

    $output .= "
            <br style="clear: both;" />
        </div>\n";

    return $output;
}