The robots.txt is
User-agent: ia_archiver
User-agent: Internet Ninja 6.0
Disallow: /
User-agent: *
Disallow: /artist/edit
Disallow: /artist/update
Disallow: /comment/show
Disallow: /comment/create
Disallow: /pool/add_post
Disallow: /pool/remove_post
Disallow: /post/atom
Disallow: /post/upload
Disallow: /post/create
Disallow: /post/destroy
Disallow: /post/tag_history
Disallow: /post/update
Disallow: /note/history
Disallow: /tag/edit
Disallow: /tag/update
Disallow: /tag/mass_edit
Disallow: /wiki/edit
Disallow: /wiki/update
Disallow: /wiki/revert
Disallow: /wiki/history
Disallow: /wiki/rename
Disallow: /user
User-agent: HTTrack
Disallow: /post
Can't say I've much experience with robots.txt other than manually adding a sitemap.xml location for the 3 big spiders (yahoo/msn/google) to garantuee they'll find every relevant link, but yours seems to:
Disallow ia_archiver and Internet Ninja 6.0 to do anything.
Disallow HTTrack from accessing anything in /post
And the big list to ANY 'robot'/spider.
Link to an image page is like:
http://chan.sankakucomplex.com/post/show/122870/c-c-chibi-code_geass-lelouch_vi_brittania
Which doesn't seem to give any problems with the rules in robots' txt.
Path to an image is:
http://chan.sankakucomplex.com/data/f5/bf/f5bfec62b8e3f5e05fc70935ac4dc5a0.jpg
Thumbnail, similiar:
http://chan.sankakucomplex.com/data/preview/f5/bf/f5bfec62b8e3f5e05fc70935ac4dc5a0.jpg
again both seems safe.
So all I can think of is that the danbooru devs added something to sniff out crawlers to prevent them from indexing every image as it would require an impressive amount of bandwidth. MSN+Yahoo+Google image search = 3.
3 * size of avarage image * total number of images = errr many terabytes?
Don't think not displaying something to spiders would be all that permanently lethal. The other way around is worse, displaying contents to bots that isn't normally accessible (and getting reported).
Good example would be expert's change.. they used to do that and you could visit google cache to find the answer you were looking for as visiting the page yourself would leave you with a "sign up to see the answer". Think they've been reported for it as they have changed their ways and instead of sniffing out user-agent strings they took a cookie based approach.
Block the cookies from their domain and you can see the answers at the bottom of the page ( I don't see much wrong in scamming scammers, but feel free to moderate my post if you consider this tip 'borderline' )
If any file is to blame, I suspect it would be /app/controllers/application.rb
but as I got 0 experience with ruby, I don't trust myself ;).
Seems to be only file that contains the string 'google' where it's not meant as an example, and handles the logic whether it should throw an error or display a page. (Though I'm sure that's old news to Ruby on Rails developers)