this is gallery post type – we recommend usage of custom meta for gallery images selection, although you can use native WP gallery, but then more advanced controls are not available …
Archive | Media post formats
RSS feed for this sectionImage post format
To use image post format simply choose featured image in „Featured image“ default custom meta box (on he right hand side)
WP native gallery example
Multiple WP galleries are rendered in archive (or taxonomy archive) view in the same view. In the single gallery post page, image groups can be view separately. Custom meta settings will not apply on WP gallery.