Well, FP, I'm more than happy to attempt to answer your original question about this photo:
Basically, I just adore Glenn's look. The wavy, mussed dark, longish hair. The handlebar moustache. The open-necked loose shirt and the tight blue jeans. I love that style in general - it would make me take a second look if I saw a guy in the street dressed like that - and in my opinion, it really suited Glenn. He looks young, confident and masculine - the lazy, casual rock star. Speaking about this picture specifically, I love his stance, the way he's focused on the guitar, and there's something cool about the way he has his back to the audience. It's a young, hot guy up on stage, drenched in sunshine, playing his guitar, watched by thousands of people - basically living out his dream. It's a great, dynamic shot.
I guess that '70s look on Glenn just does something to me. The Hotel California pictures, for example, literally make me swoon. He exudes sex appeal - the rugged, tousled look has always appealed to me.