<< Dear Frenchy, or should I call you Pilou?

Pilou is more friendly! Frenchy is more general! ;)

When I say images is not inside your file, it's that you can save the 3 images directly with your 3Dm File!

Case: View / Properties / Embed Image Data in 3DM File
So I will can see what that you do exactly! ;) (Images, position, scale...)

Of course file will be a little more big but...

If something is not well aligned :
either original scale of one image is not at the good scale
or trim is somewhere bad
or aligment is not well realised
or...

Courage! :)

Ps I am not very fluent with imperial measures - an headache nightmare! :)
Decimal is a dream in comparaison! :D

In any case dimension are not important if you have 3 images well fited between them!
You will can resize the 3D model at the end if you need some 2D / 3D drawings for Print or...! ;)