This plugin let’s you place notes on images like you know it from flickr. Via a additional box in the admin’s post edit view you’re able to place notes on images associated to the post.
Please take note that it’s currently no final release, so there might be bugs.
If you have any problem please consult the <a href="http://support.knuspercode.de/index.php/categories/2/knspr-imgnote" rel="nofollow ugc">support forums</a>
Uses the <a href="http://odyniec.net/projects/imgareaselect/" rel="nofollow ugc">imgAreaSelect jQuery plugin</a> by Michal Wojciechowski and is based on the <a href="http://www.sanisoft.com/blog/2008/05/26/img-notes-jquery-plugin/" rel="nofollow ugc">imgnotes jQuery plugin</a> by Dr. Tarique Sani.
<h3>Workflow</h3>
<ol>
<li>Add a new post or edit an existing one</li>
<li>Upload the images via the WordPress media upload tools and insert it in the post</li>
<li>If the post is new, safe it as draft and reload the page, the images should be visible in the thumbnail list</li>
<li>If the post is an existing one just hit the “Reload” link in the thumbnail list and the new images should be visible</li>
<li>Click the thumbnail of the image you want to add notes to and click the “add note”-Button.</li>
<li>Add all your notes, they will be automatically saved as you click “Save note”.</li>
</ol>

knspr-imgnote

This plugin let's you place notes on images like you know it 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.

knspr-imgnote vs External URL Featured Image