Guahan Web

This plugin uses the Flickr RESTful API to retrieve the specified number of images for the specified user for display in 
the sidebar of your WordPress blog. The display of the widget is customizable by defining the title as well as the number 
of rows and the number of images per row.

Guahan Flicker Widget

This widget will allow you to display your newest Flickr images easily in your sidebar.

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.

Guahan Flicker Widget vs External URL Featured Image