有两个挂钩可以过滤这些链接：previous_image_link 和next_image_link. 因此，我们可以为每个过滤器添加这些类：

add_filter( \'previous_image_link\', \'wpse_77296_img_link_class\' );
add_filter( \'next_image_link\',     \'wpse_77296_img_link_class\' );

/**
 * Add CSS class to image navigation links.
 *
 * @wp-hook previous_image_link
 * @wp-hook next_image_link
 * @param   string $link Complete markup
 * @return  string
 */
function wpse_77296_img_link_class( $link )
{
    $class = \'next_image_link\' === current_filter() ? \'next\' : \'prev\';

    return str_replace( \'<a \', "<a class=\'$class\'", $link );
}