This is not the official cover, but boy, does it get a reaction.
During a Tweet Q&A earlier today, Cassie said this much about the deaths in City of Heavenly Fire:
Six characters we know by name die in CoHF. That's all I will say. @JESS5698
— Cassandra Clare (@cassieclare) September 26, 2013
Thanks to TMISource for the placeholder tip!