Just to clarify why a background photo is usually not going to influence lighting, have a look at this.

I loaded a self-lit photo on a one-sided square. Floating in front of it are various spheres. With the various positions chosen, they are experiencing varying amounts of sun, sand, and sky. But we can't see any of that from the front.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)