Robert Peake

Robert

Embeds a responsive slideshow of Flickr images from any album or photoset
Shortcodes are of the format: <code>[fshow url=https://flic.kr/s/aHsiP3Xyxx]</code> or <code>[fshow photosetid=72157627847553181]</code>
The default parameters for api key, username, user_id and photosetid can have values set using the plugin options page that will then be used by default whenever they are omitted from the shortcode.

Responsive Flickr Slideshow

Does the trick

Embeds a responsive slideshow of Flickr images from any album or photoset

jmarreros

This Plugin allows to use an external image as Featured Image, this can be especially useful if you house your images in another location.
This Plugin is compatible with Nelio External Featured Image Metadata, so you can reuse your data in case you want to uninstall Nelio Plugin.

External URL Featured Image

It is what I was looking for: simplicity.

$external_image = get_post_meta( $post->ID, '_dcms_eufi_img', true );

Works well for me

Use an external url image as Featured Image for your posts, pages, and other kind of post types.

Responsive Flickr Slideshow vs External URL Featured Image