Tizionario

FlickrPhotogallery allows you to insert one (or more) image gallery (retrieved from Flickr) into your posts using a shortcode; 
You can also use the built-in widget to insert the gallery into the sidebars.
The plugin allows to retrieve: 
* Public images 
* User-specific images 
* Photoset-specific images 
* Group images
It is also possible to choose to show the image gallery as an unordered list or as a slideshow, in this case you can select the dimension of it.

FlickrPhotogallery

This plugin allows you to insert an image gallery retrieving images from Flickr.

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.

FlickrPhotogallery vs External URL Featured Image