Heureka!
It may not have been a great step for mankind but it sure made me happy.
I followed Jacub Foltiks advise and managed to insert a simple bmp-sketch in a drawing.
It turned out that MartinHanak was correct - Irfanview was the culprit and when I changed the registry as suggested I could get my scanned ribdrawing into the drawing.
I have now traced one of the ribs by sketching a spline.
How do I get this spline into a part to be able to extrude a rib?
I saved a copy of the drawing and deleted the scanned bmp to get a "clean" drawing with nothing but my sketched spline and have saved a copy of the drawing in dxf-format but cannot find a way to use this in a part-file.
