Different Image Problem for 2.5

I’ve been looking at the problems other people have had with uploading images on the new 2.5 upgrade but I haven’t seen this error yet. When I try to upload images or get/view the images that are already on the server I get this message:

“Fatal error: Call to undefined function: image_downsize() in /var/www/vhosts/midnightlights.com/httpdocs/wp-includes/post.php on line 2326”

and I looked at the code and this is what that line (in bold) says:

function wp_get_attachment_thumb_url( $post_id = 0 ) {
$post_id = (int) $post_id;
if ( !$post =& get_post( $post_id ) )
return false;
if ( !$url = wp_get_attachment_url( $post->ID ) )
return false;

$sized = image_downsize( $post_id, ‘thumbnail’ );
if ( $sized )
return $sized[0];

if ( !$thumb = wp_get_attachment_thumb_file( $post->ID ) )
return false;

$url = str_replace(basename($url), basename($thumb), $url);

return apply_filters( ‘wp_get_attachment_thumb_url’, $url, $post->ID );
}

I don’t know what this means exactly. How can I fix this?