There are two ways that I know of to do this...

1. Go to the view menu in the window that you want to import the image. Click Image plane>Import image. It is a good idea to set the window to default view before doing this.

or...

2. Just make a polygon plane and assign the image to the plane in hypershade. You can also make the plane see through and therefore easier to work with by going to the relevant viewport and clicking shading>x-ray.

Sorry if this is a bit vague. I am stuck at work and cannot check my computer. Hope this helps though!