I found the image below at the Texas History site of the University of North Texas
. It appears there as is, without any further explanation (or date).
I realize that the indentations on top of bricks are called 'frogs', but why were actual frogs being placed inside bricks?
As far as I can tell, it must have been an experimental demonstration of the 'pressed frog phenomenon' — this phenomenon being that one can place a living frog inside a brick as its being made, apply thousands of pounds of pressure to the brick to mold it, and the frog will survive. The frog won't be happy about the experience, but it won't burst. Whereas the same pressure applied to a frog that isn't in a brick will definitely cause it to burst.
Obviously the brick hasn't been heated in a kiln, because that would definitely cook the frog.
The article below from 1925 explains the science of why a frog in a brick doesn't burst. The key part of the (overly long) explanation is this sentence:
when the pressure was exerted gradually there was a tendency for the particles of clay around the body to "wall up" the body by the grains of clay moving instead of a tendency of the body "bursting" by the particles of the body moving.
However, this doesn't solve the mystery of who first decided to put a frog in a brick.
Clarion Ledger Sun - Aug 16, 1925