Yes, any fanfic posted on this forum is open game for a cover.
And having graphically skilled judges doesnt mean they look for what is the hardest to make or what took the most amount of time, because even minimalistic styles can look better than the most complex creations. What I would look for is originality, creativity, depending on how he did it the amount of possible effort that went into it as well as relevence to the story. Also Typographiy is important. It isnt just about who used the most layers in photoshop.
PS: May we use other official game art/screencaps for the cover. Or do these have to be completely self done?

Chaos is the score upon which reality is written.