This is interesting. I took these pictures to our structiral engineer here at work (we do steel construction) and while he is no expert in explosives , he did point out that if those two puffs at the corner were explosions we should have expected that the tower would be most likely to collapse in the direction of those explosions, as a weakening in that corner combined with the slumping structural steel from the burning planes would have caused a sideways collapse.
He also poo-pooed the idea that the fire in the towers had to be hot hot enough to melt the structural steel, he assumes that the steel inbetween floors is the same that we use and that the steel will slump and deform at 800-900 degrees farenheit, causing a straight down collapse, like we see.
In short, we have no clue if those puffs are explosions or not, he said it was possible, but not probable viewing the video and that if it was an explosion it was not big enough to structurally weaken the building.