I think it's ridiculous that they aren't releasing the photos, but it's established that they exist by politicians on both sides of the aisle. The simple answer is that Osama Bin Laden was struck just above the eye and the exit wound caused his skull to fail, meaning that he only had half a head left, with more than a considerable ammount of brainmatter showing.
I'm not a forensic pathologist, but like anyone else on the internet I've seen my share of severely battered corpses, and it's not like he would be easily recognizable to anyone who didn't realize that heads deform quite drastically when they're subjected to that kind of trauma.
Clearly US politicians have made the call that releasing pictures of corpses in that state is not very clever. I think they're wrong, and I think people deserve to see, even if it's gory. If you look at the killing of Abu Musab Al Zarqawi, they quite obviously held off showing the picture of him dead until they had evaluated whether it was responsible to do so or not. Judging from the pictures, he had died from the shockwave of the bomb, rather than shrapnel, which meant that his face was completely intact.
Al Qaeda have no stake in losing Osama Bin Laden. None. He's the face of the organisation to the world, and he's famous for having cheated the entire US military out of catching him on several occasions. He was Robin Hood to the fanatical islamists of the world, and they had no interest in seeing him die. He was consistently drumming up support for his organisation through audio messages, so it's safe to say that Al Qaeda had no interest in having him cease his operations.
By all accounts right now they're struggling to introduce a new leader, which certainly wouldn't be the case if he had been dead a long time. Surely, they would've planned ahead.
Al Qaeda has never needed reasons to justify new attacks, that's your very own assumption. They didn't justify their attacks in Kenya and Tanzania, they didn't justify their attacks on New York. They didn't pretend like Osama was dead to justify their biggest wholesale slaughter of innocents (which was also their biggest slaughter of innocent muslims) on 9-11, in spite of the US having tried to kill him on several occasions, so why on earth would they feel the need to do so now? It doesn't add up to what we know. If you believe that you're fighting a Holy War, and God wants you to kill yourself in his honor, why on earth would you ever need a reason beyond that, let alone one tied to a mere mortal?
The leap of faith that anyone is required to take in order to believe what you're suggesting is more than slight. It's huge. It defies history and the known capabilities of intelligence, government and military, as well as Al Qaeda itself.
Certainly though, since the CIA has shown themselves incapable of hiding every major political conspiracy that they've been a part of to date, it's definitely assured that this would come to light if it was in fact faked. Personally, given the evidence that has already been offered, I think it would've been revealed within a week if they had tried.