I think they have the located blast site incorrectly.
From the following picture, the blast site is not at the black mark next to the building and the fence -- it is in the middle of the sidewalk.
I'm becoming skeptical we are going to see the bombers in photos that are as early as when the big crowd was gathered, such as the photos analyzed here.
The bombing occurred after the big crowd had left, as it was getting toward mid-afternoon. And the bomb was left out in the middle of the sidewalk.
Therefore, I think this photo from the NY Post is important:
The bomb would have been placed about 15 or 20 feet beyond where the guy in the blue jacket is talking to his friend in the white hat, and would have been left right out in the middle of the sidewalk.
Judging from shadows and the size of the crowd, it was probably placed there only minutes after this photo was taken. From the most recent info, it seems unlikely those two guys in the photos are the bombers.
I don't think the bombers have shown up yet. I would expect the bomber was not someone who had been standing there for hours, but likely walked by shortly after this photo was taken, dropped the bag, and immediately walked quickly away. They really are going to have to focus on video from area surveillance cameras from right around the time of the bombing to try to spot someone walking quickly through the area and dropping a bag.