As Darren said, you can use whatever you want, as long as you declare it. If Krice feels that using graphics made before the challenge is okay, then it is okay, and he can declare a success if he feels he was successful. I think the original rules of 7DRL would actually allow me to work on my Vapors of Insanity during 7 days, release the result as my 7DRL entry, and have it rated in the reviews. Although for some reason it seems that nobody ever did anything like this, as far as I know.
But the best 7DRL is not the same as the best roguelike created during the 7DRL challenge. Suppose that Crogue and Drogue have been created during the 7DRL challenge. Crogue is beautiful and Drogue is ugly, and other than that, they are similar, so Crogue is a better roguelike. However, Drogue's graphics were created during the 7 days, while Crogue's graphics have been created before. Therefore, Drogue is more impressive than Crogue as a 7DRL.
The problem here is that, to make an innovative 7DRL, innovation is probably the hardest part, and it is typically done before the challenge. Having ideas, discussing them on fora, writing design documents, or thinking about the balance, all of this is serious work. For some reason it seems that people don't count it into seven days.
Maybe we should think about the 7DRL review system. Should features made before or after the challenge be scored? I am for YES. I think the purpose of the reviews is to (a) have every entry played by at least two people, (b) provide clues to the players which of the hundreds of games are worth to try, (c) tell who was successful. Both (a) and (b) are arguments for YES, (c) is for NO, but (c) also goes against the original spirit of the challenge, which said that success or failure is decided by the challengers themselves. Although I think it would be okay if the Committee can declare that a game is a failure (either because of quality, or because of being only a tiny change on pre-existing content) even if the challenger declared it a success.
From the last years' experience, the delay between the challenge and the publishing of reviews was also a big problem. Maybe we should change the rules that you need to review at least 4 other random entries to consider yours a full success? Or something like that.