You can see the image in preview if you also connect the image to Diffuse_Color. I forgot to mention that. But you should still set Diffuse_Value = 0.

Yes connecting it to Alternate_Diffuse will prevent it from interacting with lighting. This is on purpose. I don't really think its a good idea to light your background, particularly if it is a sky photo.

Consider this image:

The correct lighting for James (and anything else in the scene) is very dark, and lit mostly from behind. If I had lit the background photo with the same lighting, it would look completely wrong.

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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)