As far as I know it is not possible to download those
not linked items with HTTrack.
But there is a nice piece of software called FlashGet (
a download-accelerator ).
With that you can make some Batch-Downloads.
Get it here: <http://www.amazesoft.com/>
Configure it to your personal needs.

And for your problem I think it could be done like this :

Go to "Jobs" and "Add batch download..." and insert
URL :
<http://www.unitedmedia.com/comics/peanuts/archive/images/peanuts2001(*).gif>
From : 0
To   : 999999999 ( nine times the number nine )
Wildcard bytes: 9

This may take a very long time.
Like any brute-force method there are of course a lot
of bad shots ( not downloadable links ) processed with it.
But than you have all Pictures available in this
Directory matching that filename-citeria.