I do some drawings for a friend who does patio awnings and additions, he never gets a CAD file to start with so most times I open the (usually scanned) image in an image editor, Inverse the colours (so you get a black background with white lines) and insert it into the drawing. I then set the transparency level so I can just see it and then trace over the lines I need, scale to the correct units and delete the inserted image.
It's not super accurate but it's enough for additions and such.
hth