Ah, but there is a drawback to that procedure. It may be simple for a creator to execute (and I've done it myself), but if a person is scrolling through the DLS for items uses the 'show icons' display, then all they will see is 'generic screenshot' and not something descriptive of that the item actually is. I agree that it has been going on for quite a while as witnessed by looking for something like road signs. All you see for a thumbnail/screenshot is a bunch of road signs with no idea as to which one is the actual content.
On your own system, to correct the missing screenshot, then go right ahead and dump a generic screenshot into the folder and commit the item back. For newly created items, please don't do that.
Bill