Background and UV images are shifting ...

When I want to draw meshes or shapes on a background image, a plan in this case, the image always shifts, when zooming in and out …
I cant get enough precision to model from that plan accurately !
It’s the same with the UV texture editor !
Is this normal doctor ?

I used big scans ok, but it seems to be the same with lower res images …


nobody see what I mean ?

Is it the same with every soft that displays openGL textures and images ?
Is the question obsolete ?