In a new post to developers and publishers it looks like Valve has taken a look at some of the recent feedback regarding screenshots that appear on a Steam store page. In that in some cases they don't actually reflect what a particular game looks like, in-action. Citing its own DOTA 2 as one of the culprits. The post goes on to state that they'd like implement a rule that means any screenshot showcased on a game's store page, needs to be an actual screenshot.

And not a pre-rendered image, cinematic still, bit of concept art, or some sort of "this is what we'd like the game to look like". Naturally, as with all marketing-related bit of news these days, you could point to No Man's Sky being the reason for the change. As the Steam page for that game features images depicting events not possible in the final release version. But it's something that a majority of developers and publishers do, with a number of high profile titles featuring screenshots that are little more than promotional material from long before the game was released.

Here's the statement.

We haven’t been super crisp on guidelines for screenshots in the past, so we’d like to take this opportunity to clarify some rules in this space. When the ‘screenshot’ section of a store page is used for images other than screenshots that depict the game, it can make it harder for customers to understand what the product is that they are looking at. Additionally, we’re going to start showing game screenshots in more places as described above, and these images need to be able to represent the game.
We ask that any images you upload to the ‘screenshot’ section of your store page should be screenshots that show your game. This means avoiding using concept art, pre-rendered cinematic stills, or images that contain awards, marketing copy, or written product descriptions. Please show customers what your game is actually like to play.
For elements such as marketing copy, awards you’d like to show off, or descriptions of your Deluxe Edition, we ask that you use the specific spaces already available on your store page to put that content rather than including it in your screenshots.
Dota 2 is an example of where we were doing it wrong ourselves. We’re now in the process of updating Dota 2 to use screenshots of the game rather than artwork.