The best shot of Diablo, albeit a little hard to see, is of the scene before you enter Hell and the Wanderer morphs into Diablo. Right before he enters the portal, he looks back at Mephisto as he's talking to him. You can kind of see that his face actually has some more human-like features to his face that doesn't really resemble the Diablo head you see in the banner.
Siaynoq's Playthroughs