[Therion] Scale image

Hi, sorry to bother you with a beginner question, but cannot make it even reading through the archives. I have a .th file with centerline and .th2 file with his scraps. Now I should add to this a th2 file with an image of an old survey. I drawed walls and all objects in the scrap but do not know the scale and do not have original data of the centerline. How can I put together my centerline/scrap of known scale with this image? Do not know how to do it and I'm having dificulty understanding how the scale process is working. could you help me on this? thanks a lot Andrea Marassich