How to show link upload option on statuses updates form which converts to embedded content?
So do you want it to act like Facebook aka pulling in content from the link automadgicly?
If so then I did have some success with a alter / Ajax callback and then file read contents and pulling in an image tag at appropriate size was not the best solution but look in to https://stackoverflow.com/questions/6476212/save-image-from-url-with-curl-php 

And file_get_contents()