Indexing attached image titles for the parent post

functions.php

<?php

// Add this to theme functions.php or in a code snippet. Then rebuild the index.

add_filter( 'relevanssi_content_to_index', 'rlv_add_image_titles', 10, 2 );
function rlv_add_image_titles( $content, $post ) {
    global $wpdb;
    $results = $wpdb->get_col(
        $wpdb->prepare(
            "SELECT post_title FROM $wpdb->posts WHERE post_parent = %d",
            $post->ID
        )
    );
    foreach ( $results as $title ) {
        $content .= " $title";
    }
    return $content;
}