From CGPuppet – Preparing Image Planes in Photoshop:
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In this tutorial we will learn how to prepare image planes for automotive modeling. Blueprint images for automobiles usually have the front, back, side and top view in one image, while we, need those four views as separate images so that we can set them as image planes in our viewport. They also need to be sized properly so that we don’t have any trouble setting them up, or worse, too much offset while we are modeling.”