Nice to see you making cars for MM2!
Though the blueprints' problem is solved, let me make it clear... As far as I know, the background image in Zmodeler can be of any size (not necessarily a power of 2) but it
must be a square, its height should be equal to its width otherwise it'll look stretched/ out-of-proportion.
Now suppose you have a blueprint of a car and you separated the right, top and front views from the main blueprint into 3 different images and saved them as .bmp. Now if the width of the image in right view is 500 pixels, (could be anything, just an example) and its height is 400 pixels, and the car you're to model fits in it exactly leaving no white space in the front, rear, top and bottom then the height of the 'front view' image should also be 400 pixels. This is not a square and it will appear stretched in Zmodeler. So to make it a square, increase the height of the canvas to 500 pixels and make sure the image is anchored at the center.
In short, for having good images in ZM, leave no white space in any of the view, front, right ,etc while separating them. And make sure that the canvas size of all the images is a square.