" I have saved the background as a JPEG, "

I never spoke about the background. Read what I said. It discards your background when you save the RENDER (not the BACKGROUND**, **the RENDER) as PNG.

Regardless of what your background is made of, when you save as PNG, it is REMOVED. Your resulting FOREGROUND, saved as PNG, is ONLY the FOREGROUND.

Sorry for shouting.

"rendering both background, and forground in JPEG"

You don't render in JPEG. You export or save AS JPEG.

Try this next time, with a render you already did BEFORE you read this.

Export the image as PNG and again as JPEG.

Now go look at those two files. Same render, different file formats, different results.

JPEG doesn't have an alpha channel.

 

----- NEW INFO FOR YOU -----

Saving as PNG is what is removing your background. If you don't like that but you want to save as PNG, then don't use the poser "Background" to hold your background.

Use a one-sided square with your image attached - mount it behind your scene.

Then the square (which holds your background) is a real prop and is not going to get removed when you export or save as PNG.

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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)