If you have an image editor that will handle layers (such as Photoshop or Paint Shop Pro) the best way is to save your render in a format such as PSD, PNG or TIF which includes an alpha channel. Then you can composite the render over your background afterwards. If not, your only option is to do File > Import > Background Picture, but be warned that Poser is a bit picky and you may get a pixellated appearance. It works best if you resize the picture to the dimensions of your final render before importing.