You can do one of two things.
1) Run the scanned image through a raster to vector converter, save the conversion as a .dxf and convert that to g-code.
http://www.wintopo.com/
2) load the image into a vector drawing, or CAD program andtrace the image by drawing over it. Then save the drawing as .dxf and convert to g-code.
Personally, I would always use #2. It may take longer, but the results will almost always be better. And sometimes a raster to vector conversion will require so much cleaning up that it takes longer that manually tracing.