I want to add the photo of a book cover "Happiness is..." to this prop square image in viewport, but copy & paste don't seem to work. how can I do it so I can then move the prop with photo into the yellow square behind boy?

Copy & paste will never work like that in Poser.  You will need to enter the Material Room to apply the image to the plane prop.

good to know, but...I went into material room with the viewport open and cannot find the photo to apply it or whatever needs to be done. do you mind spelling it out for me?

drawn

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:

oh dear, the instructions seem clear but your images don't show just ? marks

sorry about reply too soon! now I can see your images!

wow, thanks, hb.