http://www.nextpowerup.com/news/31364/steam-implementing-requirement-for-real-game-screenshots-on-listings/
 
...will instead need to include actual in-game images."  So that means no conceptual art images. 

If a gallery is filled with written content or concept art, it's "harder for customers to understand" what they're buying, says Valve's Alden Kroll.

 
No, it's more like if they are required to use actual in-game footage the devs can't lie to customers about game content as easily. Pre-renders and game trailers that have zero in-game content are all crap and devs use them whenever possible. My guess is the next misdirection to get around it is that devs will put the pre-renders into their games as cut scenes and then claim they're in-game content.

Is this perhaps blowback from No Mans Sky?

It's definitely blowback from No Man's Sky

Probably also blowback from all those tablet game ports. Have you seen the crap they post for screenshots in those throw away games? Impossible to see what the game is actually like.

Will accomplish nothing.
 
Developers for a very long time have been producing bullshots of "in game" visuals. It's the norm these days, and unless valve states the images that are shown on the product page better be from the release client and some how enforce it nothing will change. You'll still have super sampled photoshop doctored "in game" images being used like that are used right now. In fact one of my fav games right now is guilty of this. It's clear as day that the screenshots of the game are from a version of the game never released to actual consumers making the visual style of the game look vastly different than what you actually get.
 
It's funny because just the other day I saw a steam review of a game in which it was purchased on Steam according to their new review standards to seperate and degrade the opinions of anyone obtaining a Steamworks game outside of Steam,but it was listed as "received product for free" negating Valve's supposed fraud review prevention.

I find Valves statements to be naive at best. They have no clear direction or real ability to enforce this.

What annoyed me recently is I had a Greenlight listing up for a year and they pulled it randomly for "restructuring" listings.

I bought Starforge way back when. I am very glad for this.