How to batch set up thumbnail for each post?

Hi, I recently installed WP 2.9. I managed to import handreds of posts to it using plugin CSV Import. So now I have posts with correct title, post content, categoreis, tags, and publish date. I am using a magzine style theme (Arras Theme 1.4) which requires a thumbnail for each post. Currently I have to use the Admin Panel and Edit and Set Thumbnail and Update each post manually. I am wondering if there is a way to speed this up since I have many posts. The pictures/thumbnails are uploaded to the Media Library. The title of each post and the corresponding thumbnail’s file name are known. Thank you.