What code shows the "random files" and "last additions" pictures on the index.php page?
How do I hide the list of categories on the index.php page? If there is not feature, what code do I need to delete to make it go away?
Try to keep your postings in the correct categories. And try using the search function before asking a question which has been answered before already.
Disabling categories doesn't make sense - your visitors won't be able to browse your gallery. Anyway, if you must disable it: in the same place where you disable the random meta album you can disable categories as well. As Hein suggested: search the board and RTFM.