About the blueprints, there re a few things to keep in mind to get them perfectly aligned in Zmodeler. First, select a 'good' blueprint like this one:
Notice that this blueprint contains all the views of the car, you may not find all views in the blueprint of the car you're to model, but that's fine upto a extent. You must have the front, back and one of the side views at least.
Next, to start separating that single image into multiple images, (front, back, left,etc..) start from the front or back view. Here I started from back. Carefully move the selection so that the car exactly fits inside it (don't worry about the selection not being a square, it'll be made to a square later).
Now once you've selected the car from back view, copy this selection (I'm using Paint.NET to do this) and create a new image whose canvas size should be exactly equal to the size of this copied image. (Paint.NET does that automatically, just press Ctrl+C to copy, Ctrl+N to make a new file and a window will appear asking the canvas size, let it as it is without changing any values and press Ctrl+V to paste in that new file)
Now do the same thing for one of the side view: I select right view.
Now as in this blueprint the back and right views of the car are exactly aligned, I just moved the selection horizontally to select the right side of the car.
Make a new file and paste this selection into it, just like the previous one. Do the same for front and back views.
Now you have back, front and side views separated so its time to make them square so that they don't appear elongated in Zmodeler.
Go to the back view and note its width in pixels. In my case, its 137 pixels. Now go to the right view and note its width. Its 290 pixels. As we didn't change the height of selection and the blueprints were perfectly aligned, their height will be the same.
To make these images square, adjust their canvas size. The right view is 290 pixels wide so the size of back view should be at least 290x290. Go to canvas size and make the back view 290x290 pixels with its 'anchor' in the centre.
Do the same for right view, make its canvas 290x290 pixels. (Canvas size will be different in your case and not 290x290 pixels, just make sure that the
width of the side view should be the canvas size of the
front/back views
and width of the side view should be made equal to its height by adjusting canvas size.)
Now these images are ready to be used in Zmodeler.