That is not a picture of the bomb. If you look at the railing next to the mailbox in the first image, next to the blur in the second image, you see the railing is actually wrapped around the mailbox.
The explosion happened behind that barrier, not in front of it. There’s no way anyone who was helping to clear barricades to reach victims could have bent that railing in that manner. It had to have been done by the explosion.
But they are looking at ground zero, just right behind those poor people.
If you’ve looked at other photos, people had pulled back the railings almost immediately to get to the people so I’m not sure I would read too much into the position of where it is.
Also not a big explosive charge. The street signs to the left and right of the seat of the blast don’t seem to be bent. I’d have expected them to be bent/folded away from the blast.
This and the white smoke/orange flash says ‘gunpowder’ to me, not something with a detonation wave.
Still this is all speculation. By now the authorities know well what the charge was from the residue. They just haven’t shared the info with the public.