Time for yet another game of "assign a face to the character". In this case, though, it's actually "reassign a face to the character".
Originally, I'd noted that I'd come to associate Dolores O'Riordan's face with that of Zull's. However, after watching some music videos by the Canadian singer Alannah Myles, I've now come to associate her likeness with Zull instead. Frankly, her physical features and overall demeanour just synch with how I envision Zull better than O'Riordan's; her eye colour is even similar to the green-gold of Zull's eyes.
I still imagine Zull with an Irish accent, though. =P