Use the picture tool to insert the background picture on your plan. Its a good idea to place some hotspots over the picture to refer to at points of reference eg. bottom and top corner of known walls (x2). Select the picture, go to Image / Align view. Follow the prompts first picking a point on your plan (at wall intersection) then on the two points in the photo (base of wall, top of wall), then the second point on the plan followed by the last 2 points on the photo. A dialogue box appears, put in the relevant heights, and a camera should be placed on your plan. Select the camera then go to 3D view. Then curse as it didn't work properly. Does that make sense?