This is how you import a background image in P6, I imagine it works the same in P7.

From the menu bar's Figure menu, select Import > Background Picture. You will then be presented with a File Open dialogue box where you can brows to the image you want to use. Select the image and click the Open button. You will probably be presented with a message "The background width/hight ratio is different than the current window. Change window to match?" Answer "Yes" to this. The picture will then be added to the background. **If you don't see the picture at this point, open the Display menu and make sure that "Show Background Picture" is enabled.
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It's probably best if you resize the image to be the same size as your desired final render size, before you import the image. This is not compulsory, but it can improve image quality.