add_action( 'cred_save_data', 'tssupp_make_featured_image', 10, 2 );
function tssupp_make_featured_image( $post_id, $form_data ){
 
    $image_field = 'pictures'; // edit field slug
 
    if ( in_array( $form_data['id'], array( 123 ) ) ) { // edit form ID
 
        // get the images 
        $images = get_post_meta( $post_id, 'wpcf-'.$image_field, false );
 
        // get the attachment ID of the first image
        if ( is_array($images) ){
            $first = attachment_url_to_postid( $images[0] );
 
            if ( $first > 0 ){
                // set the featured image
                update_post_meta( $post_id, '_thumbnail_id', $first );
                // delete the first custom field image
                delete_post_meta( $post_id, 'wpcf-'.$image_field, $images[0] );
            }
        }
    }
}