WHDH TV in Boston was sent a pair of photos, which they have forwarded to the FBI. The first photo, taken just before the blast, shows a barrel-shaped object on the sidewalk. In the second photo, blurred due to its graphic nature, the object is gone.
The photos were taken roughly an hour apart, so the position of the security gates probably doesn’t tell us much. But the debris on the ground is pushed out in a roughly circular pattern centered on there the object in the first photo rested.