Nevermind, was able to get it working.

Logged in to Tumblr & used an add-on called "Export Cookies" on FireFox to
produce a cookie.txt (all domains, because why not :P) with the needed
cookies.

Additionally, used a plug-in called Linkgopher on Firefox to manually grab
dozens of links at a time from the ___.tumblr.com/archive pages; and input
that into a text file for HTTrack to use.

the settings I used after that were:

+*.gif +*.jpg +*.jpeg +*.png +*.tif +*.bmp
+*media.tumblr.com/*
-*avatar*
-*ios*
-*android*

these allowed pictures to be downloaded & prevented other irrelevant files
from being downloaded (as well as all the user profile pics).

make sure you go to Links & check "get non-HTML files related to a link, eg
external .ZIP or pictures"

Be warned, this takes forever & takes up a lot of space depending on the type
of tumblr blog. 

I've also found that some pictures seem to be getting downloaded multiple
types at different resolutions, but I haven't found the reason yet. They're
not linked in the pages as far as I can tell; but its not a huge deal.

Hope this helps anyone looking to download a Tumblr blog. I know the scan
rules need some work; so if anyone has some improvements to add on, please do!