There is a long history for photographic products in the USPOD/USPS. I'm working on research for this topic and have been for some time.
Here's a short synopsis. All images are from my collection.
The earliest examples of photographic use by USPOD was in essays. A good example is the following photo essay. This is an essay as the vignette was originally proposed for the two cent, as shown here, but ended up being used on the five cent. I've included the stamps as well as the essay to show what I'm referring to.
As time progressed, and photographic processes became more advanced, the USPOD experimented with use of photography. The following is a composite photo essay, not adopted. This was meant to be the three cent stamp of the 1922 series, a copy of which is included for reference.
Another use of photography was to provide the artists with an image of the required elements. This was done in the following photographs for the National Parks series of 1934. I've provided a couple of the images, along with the issued stamp, to illustrate this. I call these source photos and consider them essays.
As part of the vetting process, the USPOD began realizing it was more cost effective to take photographs of completed proposed designs instead of creating multiple copies of designs. This resulted in photographs which were absolutely essays, such as the following images for the 1953 Trucking stamp.
As further advances in technology brought the costs down, the USPOD started sending photographs of upcoming issues to the newspapers, philatelic press and cachet makers to get the information about the new issues out. These are called publicity photos. From the 1930s through early 1950s, there was no copyright or use indicia; afterward, through the 1960s, this is included. For some reason, in the 1970s, USPS stopped putting a copyright on the photos, but continued to issue them. In the late 1970s, the copyright date only reappeared on most issued photos. Additionally, there were two other formats used: Photo transparencies and projector slides. One of the problems with classifying these are the differences in designs between the time the "publicity" photo was released and the stamp issued. If the differences are significant, I call them photo essays. The following are some examples which are strictly publicity photos in the varying formats.
The first is the publicity photo for Scott 1036. Note the text above the photo. Included here is a stamp for comparison. The photos also do not show perforations until Scott 1438 unless it is a setenant issue, such as a block of four.
The first stamp which showed perforations also dropped the copyright and use indicia. This is Scott 1438.
Sometimes, with the transparencies, the copyright information was on the sleeve, as shown in the following release for Scott 1767a.
From the 1970s through the mid-90s, photographs were usually black and white while the transparencies were color. Around 1995, the photos became color and black and white started to fade from the scene, although not completely. The Texas issue of 1995 is a good example, providing an essay (value as 00), the color transparency and a color photo as well:
By the mid-2000s, everything was being done by computer and images were then being sent via email. The era of photography as a means of promoting stamps, from design to publicity, was ending. The last issue I have been able to find with a photograph issued for publicity was the one issued for the setenant block of four for the U.S. Marines, Scott 3961-64. If anyone has any AFTER that, please let me know and I'll include it in the database I've compiled.