Hi Michael, this is how I position Background images at the moment.
The problem however is that quite often the background image I have (let's say a Cad plan printed as a PDF, then saved as an image)
does not have a precise length and width.

The precise information I have is the drawing itself. So in your exemple, with the car, it would make more sense for me when placing the image if:

- I drew a line that is 4 meters long (or however long the bottom edge of the chassis is)

-Moved the image by clicking on the image on one point of the equivalent line at the bottom of the chassis, then draging and snapping the the equivalent point on the line.
-Then scale the image by clicking on the same point again, clicking on the other extremity of the bottom edge of the chassis (on the image) and scaling down, by selecting again the other equivalent point of the line.
Just like I would If I had a reference nurbs object, and I wanted to scale down another object to fit exactly to it.

Basically, sometimes it does not makes as much sense to use the edges of the drawing as reference to place an image, it would be better if I could click somewhere on the image.
Maybe it is not clear, if I find a small break today I can generate a gif showing what I mean.