Karrde

Displays thumbnails from Flickr based on the user/group/tag you provide.

Flickr Thumbnails Widget

Displays thumbnails from Flickr based on the user/group/tag you provide.

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.

Flickr Thumbnails Widget vs External URL Featured Image