Use a picture as background in any 3D window

To set a background picture, you need to have a 3D window active, then switch the window with SHIFT+F7 and set the BackgroungPic button to 'on'. A Load button should appear and let you choose the file you want as background. Once selected, switch the window back to a 3DWindow with SHIFT+F5.

NOTE: The background picture appears only if the view is set to 'ortho mode' (and you choose the top, side or front view) or if you activate the camera view (NUMPAD 0).