I've never been able to trust what the firefly render is ever going to do with BG pictures. P6 has it as a glitch. Period.  People ALWAYS say 'Use auto-fit" and it never works consistantly for me. PERIOD.

The best thing to do is do everything as above, and save it as a PNG file and do a copy of the BG in an image program and paste the PNG as a foreground on it.  If the BG renders correctly, save it as anything you want.

At least the P6 glitch is not a deal killer like P7, which works like P6's unpredictability EXCEPT the BG picture is always there visible EVEN WITH PNG's and TIF's.  If the BG is messed up in P7, your render is defective.

After P7's uselessness, I can put up with messed up BG pictures, that can be worked around.

I cannot save the world. Only my little piece of it. If we all act together, we can save the world.--Nelson Mandela
An  inconsistent hobgoblin is the fool of little minds
Taking "Just do it" to a whole new level!