It won't try to remake thumbnails that are already in the database. Did you run gallerymakethumbs.php with the --flatten option enabled originally?

Edit your functions.php and comment out the line:

unlink($thumbname);

in function makeThumb. Then try posting a new message with an image attachment. Go to your /tmp directory and see if a thumb_{random stuff} file was created and is greater then 0 bytes.

Also, are you running in safemode?