I know, yeah ... I debated whether to go with Erik Wolpaw and Chet Faliszek (the writers of Portal) or with Kim Swift (whose game Narbacular Drop originated the concept of portals). I think there's probably justification for either one, but I can see the argument going with Erik and Chet since these are, after all, BOOK covers. And on most of the other covers I went with the writer and not the designer, so I probably should have been consistent.
I think, ultimately, for this particular game, the initial concept of portals was such a fantastic, original idea that I wanted to give a nod to that by crediting its designer.
All right, that's a strong justification and I thank you for elucidating me. Like you said, these ARE book covers, so I still think it's the writers who should be credited as the novels would only basically keep their scripts and a few mentions of gameplay, but it's not a big deal. I'm happy enough with the explanation you gave, it was well thought out!
I like the lack of detail. It's gives it a quasi minimalist feel. It's very fitting for book cover art and I think it works well with Portal. You should consider entering this piece in to the welovefine.com Portal contest.