In my opinion the bag at the curb is not the bomb, it appears to be a bag possibly holding water bottles that the lady behind the bag was handing out to runners. (she is holding many bottles in her hands)
I think the bomb was set near the tree (near the big guy next to the tree) and behind the single row of people standing at the barriers. The explosion and force blew several of the people near the mailbox into the barrier which caused the 90 degree bend in the barrier. (force and body weight slamming into the barrier and the mailbox looks untouched because there were several humans that took the blunt of bomb instead of the mailbox)
many pictures of the fagor pressure cooker (has the same bottom stamp as the fagor brands) http://www.myfoxatlanta.com/story/21996004/exclusive-photos-boston-marathon-bomb