you should have a look to this blog post : http://www.maverick.it/tech/create-thumbnails-using-wordpress-built-in-functions
then, if you can’t use a remote image, just download it as a temporary file on your server, process and delete it.
One your image created, you should then insert it as an attachment (http://codex.wordpress.org/Function_Reference/wp_insert_attachment) and set it as the post thumbnail (http://codex.wordpress.org/Function_Reference/set_post_thumbnail)
I hope it will help.
Cyril.