Spencer assumes thusly that the laptop was never at the dance, and Ian put it there after she left, and immediately confirms that the Kissing Rock footage is gone forever. But then, of course, there's more: A new message from A, in which we see Alison running away from the barn after the Spencer fight but before the Toby run-in where she got the sweater, and a note: Watch your backs. I didn't. - A. There's a shadow behind her, even, but instead of assuming it's Toby they just know it was the murderer.
But I mean, really that picture is all of them, and the shadow that's chasing them is Alison, because they are all sort of Alison, because she's the only other person who's solved that Girl Dilemma, just in really fucked-up ways. Which she passed onto them, like a Professor of Grossness:
Alison was the one that helped Aria discover Byron's extracurriculars, breaking down the generational wall between child and parent in a way that can't be rebuilt and which Aria's still trying to climb and figure out.
Alison was the one that taught Hanna the worst possible way to get hot, using behavior mod techniques that would make Cesar Milan proud, to make her the next Alison.
Alison outed Emily in the greediest possible way, tempting her out onto a limb and then jouncing it with all her might, and now that Maya's replaced Alison (maybe literally, somehow, if you keep an eye on those blonde wigs) and been taken away in turn Emily is basically nonfunctional, just like Ali would want.
Allison needled Spencer about Ian while sleeping with him, which turned the entire Hastings household radioactive and brought the Spencer/Melissa war to a deadly pitch that is still knocking out casualties left and right.
She even managed to get poor Jenna blinded, in a way that had to do with the parts of her own relationship with Jenna's brother we still don't entirely understand.
And what it comes down to is what both Caleb and Hanna were saying, in their different ways: Just because you have all the power in the world it doesn't make you feel any more powerful when you can't use it. The difference between oil or money and being a human commodity is that oil or money don't feel like shit about it. Ask any drug dealer the number one worst thing about their job and they'll tell you: I have all this money, and I can't spend any of it.
All Alison really did was show them just enough of the prisons they're in that they couldn't ignore it anymore. And then forgot to give them the key.