OK, remembering I am using Poser 2014, all I have to do is, with the 'show background picture ticked', click the materials tab, then in the materials room tick advanced. Select background in the object tab and then click on the BG image box on the right. In that box click image source and point at the image that you want to use. When you click OK Poser automatically connects the output of this node to the background and the picture should appear in the preview window.

The upside of this method is that the background sits in the back of your preview and stays there facing the camera as you change camera position. The downside is that you cannot change the parameters as you can with a cloth plane but is does not remain facing the camera meaning it has to be placed before you render. Don't forget if you are using background you need to select render over background picture in your renders setting otherwise it will not appear.

 

 

I use Poser 13 on Windows 11 - For Scene set up I use a Geekcom A5 -  Ryzen 9 5900HX, with 64 gig ram and 3 TB  storage, mini PC with final rendering done on normal sized desktop using an AMD Ryzen Threadipper 1950X CPU, Corsair Hydro H100i CPU cooler, 3XS EVGA GTX 1080i SC with 11g Ram, 4 X 16gig Corsair DDR4 Ram and a Corsair RM 100 PSU .   The desktop is in a remote location with rendering done via Queue Manager which gives me a clearer desktop and quieter computer room.