add_filter( 'genesis_get_image_default_args', 'prefix_stop_auto_featured_image' );
/**
* Stop Genesis archives from using first attached image as fallback when no featured image is set.
*
* @param array $args Default image arguments.
*
* @return array Amended default image arguments.
*/
function prefix_stop_auto_featured_image( $args ) {
if ( ! isset( $args['context'] ) || 'archive' !== $args['context'] )
return $args;
$args['fallback'] = false;
return $args;
}