In Gregg's autobiography, he said they were all in a foul mood the day of that photo shoot, and the shots all showed it.
Then Duane saw his dealer walking across the street; while the photog messed with a setting, he ran over, scored, ran back, and got into postion just as the shutter clicked. His hand is closed around a bag of smack. That's why they were all laughing.
And yeah - one of the all-time great live albums (or albums, period).
Peter