I think it may be from a photoshoot Shatner did for TVGuide as part of the 30th anniversary (when they did the "Four covers/four captains" thing). He was featured in a quickie article where he posed with various "ladies of Star Trek" or something. I remember thinking the collar looked nothing like it did in any of the movies:
Click me.
I have the issue somewhere, but maybe someone with faster recall can check the issue and the other photos it contained.
Hrm... I guess unless the photographer steps forward this one might be a mystery for a bit.
Photo shoots like that usually escape the research, though, unless it happens to be something I'm aware of from my personal knowledge. Those kind of 'unofficial' things get obscured over the years, more often than not. Sometimes I've simply forced to make an educated "most likely" guess as the actual facts have really faded. Like with a few recent Frakes/Riker pics. I'd not have known where they came from without the knowledge of WillsBabe who identified them (and later provided me more from the series). In cases where I'm truly, truly boggled, I tend to title them "unknown" but sometimes, I make a leap of faith, heh.