You should be able to import any psd and place it behind your puppet in the scene.
Then use the transport controls to position it.
I thought I tried before, but didn't work.
I was able to import a PSD, but I noticed it was identified as a "puppet". That feels wrong, but ok. Then, when I tried to drag it into the timeline, I got an alert that said there were items missing.
Then the app crashed. But I can try that again with other PSDs.
One thing: "transport controls"? I actually haven't been able to figure out how to size / move puppets. You'd think this would be straightforward, but I've been scratching my head.
Ch probably calls every psd file "puppet." I don't recognize that error message, but when I said you can import any psd file I probably overstated it.
Try importing a psd with only a single layer for your background. You can also add a background layer to your puppet file, but it must be separate from the character or it will warp with the puppet.
I think importing separate files is more flexible; you can then import more than one if you want to have various items in the background. Drag it into the scene, make sure it is at the bottom below the puppet, select it and look to the right for the transport controls.
Thanks for your help. I did find the controls after I read another thread where you answered someone else's question about it. However, I wouldn't mind being able to simply click / drag / resize with the mouse rather than keep guessing numbers. Oh well, hopefully that comes later.