Its just artwork, guldan is done in NightholdI totally support this. Think about it... First Gul'dan severs Illidan's spirit from his body in Black Rook Hold, then he takes Illidan's body to the Nightwell for some undetermined reason (anyone with Beta access please shed some light if his reason is stated outright). Lorewise, it's likely Illidan still has the skull on his person, but I think it isn't necessary for this story to unfold. When Illidan absorbed the powers of the skull, he also absorbed a portion of MU Gul'dan's spirit as well as his memories. AU Gul'dan probably knows this (having spoken with Kil'jaeden) and he needs those memories in order to find the Chamber of the Eye and subsequently the Tomb of Sargeras's avatar.
The Nightwell is made up of pure Arcane from the Eye of Aman'thul (primordial creation energies and time magic), so it'd be the perfect source to resurrect MU Gul'dan using Illidan's body (now without Illidan's pesky spirit to interfere). Obviously things are going to get mucked up, and with Xe'ra's help, we'll find Illidan's spirit in the Twisting Nether again and attempt to get him back in his own body before AU Gul'dan succeeds.
Something tells me Gul'dan's story isn't ending at the Nightwell, especially when we have artwork showing Gul'dan and Illidan squaring off on what is likely the Broken Shore.