I've heard the term before. And I've looked it up in a number of ways.
I'm talking about the athlete's head shot that appears alongside his stats if it is shown quickly. Not necessarily the animated ones they show on Monday Night Football. I'm talking more about still photos, very small ones. Ones that are typically used throughout a season, across a variety of media.
For example, at the start of an MLB season, if a player will now be playing with a new team, frequently they will show this "stock" photo on file from last year, and the new uniform/hat will be photoshopped onto it.
I've heard the term. It's certainly not 'stock'. The term 'media guide' comes to mind, but there has got to be a better term for it.