OK. At this point I think I'd try the following.

1. Remove all nodes from the background.
2. Resave the scene under a different name.
3. Save, individually, all the figures in the scene to your figures library.
4. Close down Poser.
5. Run something like CC Cleaner or just the standard Windows Disk Cleanup to remove all temp and cache files.
6. Reboot your machine.
7. Reload Poser. Try opening your saved scene and render. Is the background still doing funky things?
8. If it's still messed up, start a completely new scene and import the figures you saved to the figures library - this should make recreating the scene relatively painless. Render after adding each character to check whether anything strange is happening.
9. If it all loads okay, then you should be able to attach your background image to the background node and render.
10. If it's still doing weird things, then a tech support ticket with eFrontier would probably be your next move.

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire