Carey Zhou

Handles uploading, modifying images on Flickr, and insertion into posts.
上传和编辑 Flickr 账户中的图片，并且在博文中插入Flickr账户中的图片，利用Flickr做图床。
Inserting images into posts could never have been easier. Simply click on an image from the added ‘Flickr Manager’ panel on the edit page and it will automatically be inserted into your post.
本插件由 “WordPress Flickr Manager” 更新而来.

WPFlickr

Handles uploading, modifying images on Flickr, and insertion into posts.

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.

WPFlickr vs External URL Featured Image