You need to first download those external images then you can attach and set them as the post thumbnail.

To download and attach you can use the media_handle_sideload() function.

I wrote a plugin that will search through all your posts and pages and download and attach any external image into the media library.  It also has the option of setting the first image as the featured image.

http://wordpress.org/extend/plugins/media-tools

Create post thumbnails from images stored outside of uploads directory
When going to a JPG photo folder OS C drive and opening it to view photos in an icon mode the icons only show a generic picture. If I click on the icon then the actual photo comes up. It seems like this problem just started happening. In the past I would open a folder and view icons and the photos were all there to view at once. I don't believe I made any intentinal changes. Thanks for the help. 
