Open "includes/functions.php" and change this line:
"thumbnail_openwindow" => get_thumbnail_code($image_row['image_media_file'], $image_row['image_thumb_file'], $image_row['image_id'], $image_row['cat_id'], $image_row['image_name'], $mode, $show_link, 1),
with:
"thumbnail_openwindow" => get_thumbnail_code($image_row['image_media_file'], $image_row['image_thumb_file'], $image_row['image_id'], $image_row['cat_id'], $image_row['image_description'], $mode, $show_link, 1),
Greets Jan