On the Wordpress support forums there is a response to a similar issue stating..
In 3.0, we moved wp_load_image() from wp-admin/includes/image.php to wp-includes/media.php. Thus, it looks like you're running 3.0's wp-includes/media.php, but 2.9's wp-admin/includes/image.php.
I would re-upload wp-admin and wp-includes entirely."
Your issue could be that the version of the wp-admin files you re uploaded was different from the version you had on the site.