I think I explained my reasoning. The argument that taking client photos at the boss' request isn't your job seems weak, although I can imagine some facts that might make the argument a bit stronger. On the other hand, if the employer asks a gardener to do a Mona Lisa knockoff, an argument that the painting task was a side deal outside of the gardener's normal duties seems more reasonable.
Seems like a weak (or overly contrived) distinction. If the poster is a bookkeeper, is a photo outside the scope? What if she's asked to paint a portrait instead of a photo? What if she's a P.R. person? Or a horse trainer?
The test for "within the scope of employment" generally includes analysis of the "kind of work plaintiff was employed to perform." Whether or not the work is a photo or a painting is besides the point, unless, of course, the person was hired to do one but not the other.
Likewise for your "your boss asked you to do it so its within your scope of employment" argument. It isn't "weak", as you suggest, it's the law.
Bottom line, if you ask your bookkeeper to shoot some head shots of your clients, then the bookkeeper is likely the author of those photos.