Thanks to you both!
And here I was thinking there was a nice clean method just like the tutorials suggest
@emc_deux: Do you crop the respective blueprints (front, top etc.) and apply them one by one to the respective windows? I did so once but I had to use the same width and height for the four images (like for example the size of the top view applied to the front view etc. resulting in a lot of white background) to make them appear properly in Blender. This is why I thought uv-mapping them was easier. And also more correctly.