This is an important image because it clearly shows the blast pattern of the explosive which was placed in front of the fence beside the mailbox, as you can see by the locations of the fence and other debris just after the blast. There is no question where the detonation took place. In fact, a bystander has recently given an interview claiming his life was saved by the mailbox. If the bomb had been place behind the fence and behind the mailbox on the sidewalk, no bystanders could make that claim, only runners.
I am not saying that bag was the bomb. It may have been, but I am saying that bag is in the location the bomb was detonated. It went off IN FRONT OF THE FENCE and BESIDE THE MAILBOX.
Part of one section of fence is wrapped around the mailbox. The other section is blown back into the crowd and on the sidewalk as you see. Were the bag to be partially under the fence as appears in the photo of the bag in front of the fence, that could easily be the result.
That is NOT the location the Feds claim they show Dzhokhar Tsarnaev dropping his pack. Had the bomb gone off where he was and where that image shows him CARRYING his pack, then the fence would have been blown into the street.