The most common usage of 本家 you’ll hear in anime is “main house”, as in the place from which distant patriarchs arrange marriages and summon disobedient underlings for punishment. On Pixiv, it generally refers to artwork created officially for a book, game, show, or a Youtuber, drawn by a professional artist.
It’s a good way to go trolling for some really well-done images, generally less explicit than the fan-art. In fact, pretty much all of these ended up outside the NSFW tag, and even the ones that went in were pretty tame, with mostly-mild violations of my PEEN (pose, exposure, emphasis, nipples) rule.
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Sometimes you have to double-click to enter text in the form (interaction between Isso and Bootstrap?). Tab is more reliable.