This one had me fooled for a while, because of the "Theatre" sign on the right, but I'm pretty sure now that this was taken from the intersection of Third and Main looking North, as the artist's rendition says. It's neat to see the bicycles (one being ridden, one parked) in the picture and we have trees now!
(You can view a larger version of the pictures by clicking on them)
There was an "Odean Theatre" at 436 Main in the early 1900s but that would have been a block up the road. Taking a look at the picture today below of the Dickens Building (year 1881 on the sign), you can see where the picture above begins, also with the Dickens Building on the immediate right. This is indeed the 300 block of Main Street, Longmont.
And, what the picture looks like today:
In the original above where you see the Hamm-Weller hardware store sign painted on the brick ("Hardy Implements"), this would have been on the white wall below on the near right, in today's picture:
The great book "The Longmont Album: History and Folklore in the St. Vrain Valley", by Betty Ann Newby, has a good description and a picture of the Hamm-Weller store, which was located at 322 Main.
The Hamm-Weller building today:
Now, to figure out what year this picture was from!