What's the story behind the Altair fanart in episode 20?
I just found an interview with Tomoyuki Arima, a member of the art direction team of the show on Pixivision.

Relevant part:


  "--By the way, I heard some illustrations that appear on Piclive were actually posted on pixiv. Is it true?
  
  Arima: it's true. We commissioned those pictures in order to add them to Piclive. I don't know much about those commissions, but the original illustrators were allowed to post their work on pixiv, I think"


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So, to answer the actual question: The pictures weren't planted and the they didn't contact the owners of existing artworks either. They simply asked a bunch of pixiv artists to create some artwork for them which they could put in the anime to give Piclive a more realistic Pixiv feeling, and the artists were allowed to upload these onto their own Pixiv.