The correct procedure for adding an image to a flat plane prop is loading a one-sided square to your scene first.  Navigate in your browser to the image you would like to use for the square prop and determine the X- and Y-dimension in pixels of the image and apply it to the x- and Y-scale of the prop.  In my example below, the image measures 378 x 400 pixels.  

                                                                                                            

Enter the Material Room, and either Drag & Drop the image onto the background or create a new image map node and navigate to where the image is located.  

                                                                               

Connect the Image-Map node to the PoserSurface as shown below:

                                                                   

Return to the Pose Room and you should see an image before rendering as illustrated below:

                                                                                         

If you need to rescale the square, use the channel dial below: