Version 0.91 update:It works for non flickr.com images,have fun! Get Flickr Thumbnails plugin can get thumbnails for photos on your site that from flickr.com,you can choose recent or random mode,and it will link to your own post.

Get Flickr Thumbnails

Get Flickr Thumbnails plugin can get thumbnails for photos on your site that from flickr.com

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.

Get Flickr Thumbnails vs External URL Featured Image