Igor Benic

Multisite Featured Image changes the box of the featured image for all sites. It can be used also in a single instance of WordPress and not a MU installation.
This plugin changes the box in the way that the default pop-up is replaced with the classic Media Uploader where you can choose images. Images From URL do work.
For a greater Multisite experience you can combine this plugin with the <a href="https://wordpress.org/plugins/network-shared-media/" rel="ugc">Network Shared Media plugin</a> and have a complete Media Sharing site.
Read about it on this <a href="http://siderrz.com/wordpress-multisite-shared-featured-image/" rel="nofollow ugc">article</a>.
<h4>Features</h4>
<ul>
<li>Using WordPress Media Uploader to set Featured Image</li>
<li>You can set an image from URL</li>
<li>Images from URL will be downloaded into the WordPress Media and it can be used for anything else inside WordPress</li>
<li>Downloaded images are saved in every registered image size </li>
<li>You can set the Main site on which you will download the images in a MU installation</li>
</ul>

Multisite Featured Image

Multisite Featured Image changes the box of the featured image so that you can use the classic Media Uploader also with Network Shared Media.

Mohamed Tawfik

This plugin let you add a featured image to a post using external URL.
You can use images from Flickr, google images… etc just in one click. 
No need to download the image and re-upload it again.
The image will be processed and thumbnails will be generated just like if you uploaded the image your self from your PC.
It is working with native and custom post types.
It is available now in two languages:
1- English 
2- Arabic
You can translate it to any languages.

WK URL 2 WP Thumb

This plugin let you add a featured image to a post using external URL.

Multisite Featured Image vs WK URL 2 WP Thumb