When you set a background image on a window, it is only visible if the window is perpendicular to one of the three axes. That is, if you use the 1, 7, or 3 numkey to set the view the background image will be visible, but it will vanish the instant you rotate the view.
The background image is set by clicking the VIEW menu at the bottom of the window and selecting Set Background Image.
I usually use three blueprints, the standard front/back, top, and side view. I find that it makes things more easy if each blueprint image is square (same number of pixels wide and high), and if each image is the same size as the others (yes, this means the front image is a tiny diagram in the center of a blank picture).
When I do a spaceship, I usually have the side view with the nose to the right and the tail to the left, and the top view with the nose to the top and the tail to the bottom.
Be sure each image is “aligned”.
 on the top view have the centerline of the ship on the top-bottom centerline of the page.
 on the side view have the centerline of the ship on the left-right centerline of the page
 on the front view have the ship centered on both centerlines.
I split the 3D area in Blender into three windows, usually with a half-sized window on the left and two stacked quarter-sized windows on the right. I set each to a different background image: front, top, side.
View:Set Background Image will allow you to move each image so their centerline coincides with the appropriate axis. If the images are too large or too small, you can alter it with the SizeX and SizeY controls. Be sure to use the same value in both controls.