now why-oh-why would Andy's look-a-like take a proof positive photo with the individual who he would be impersonating in death?

I don't know, I would want one. I wonder who took the photo?

Why would you want one, especially if it could come back to haunt you and jeopardize you're status as legend?

It's like a smoking gun.

If it was me, I would want one. It's more than evidence, it's a "gotcha!". If the photo really is what we think it is, its existence suggests to me that he is not planning on returning.