I don't have Poser 11 so I am not sure on the fix to this problem but assuming it is the same as Poser 2014 do you have 'show background picture' ticked under the display tab. If that does not work, how are you importing the picture (might be a silly question but worth asking).

In earlier versions of Poser I have found adding a high res square from primitives and the adding an image map in materials to be a more flexible method but I accept if you are new to Poser this might be a bit advanced.

 

 

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.