Dragon Ranger @ 3dbuzz
This is going to get a little involved.

To start off set your viewport to 4 views and wireframe.
Now with your mouse over the viewport hit the “d” key (no shift, space, alt or control) this opens up the Display options.
You will want to uncheck the “Apply Operation To All Split Views” that way you will not place the first picture in all 4 views.
Using the Spacebar and the “n” key you can set control to which ever viewport the mouse is over, you can tell if it worked because the text turns yellow.

Choose the viewport you want to place an imageplane in then in the display options select the background tab.
We will stick with importing an image for now.
Check the “Display Background Images”
To the far right of the “Filename” label there is a Document search button ( it looks like a page with a small arrow on it) click this and locate the image you want for your selected pane.
You should now see your first image, but you now have a problem in that when you navigate around that viewport the image remains stationary while everything else moves around.
To fix this click the “3D Viewport” tab and uncheck “Automatically Place Image”
You will also see image offset and image scale fields, we will get back to them.
repeat this process for the remaining viewports. These images do not appear in the perspective viewport.

Unless something went terribly right your imageplanes will need adjustments.
Place a box in the viewport and adjust the width and height to match which ever imageplane you are going to use as the “Master”.
choose one of the “Slave” Viewports using the Spacebar + “n” key combination. Now is the time to use the Image Scale Fields to adjust the size of your slave image. It is probably best to adjust the image uniformly so that it fits into the bounding box (you may need to play with the image offset fields as well.
Now go to the box and adjust the remaining dimension to fit the second image.
Repeat the adjustments to the third image if you have one.
When you are through you can remove your box as it is no longer needed.

I hope this helps.