How to convert boundaries in image to polygons in QGIS

Question

I have an image from a cities districts and their boundaries and I need them as polygons in QGIS.

I tried "Raster -> Conversion -> Polygonize (Raster to vector)" as described here, but that vectorised every single pixel.

Answer 1

Since the number of districts is quite low, you could just digitize them manually. That will probably be faster than anything else you try.

Otherwise, as Mapperz commented:

If you cannot get the data in a more appropriate format somewhere else, get the Vector SVG file and convert and georeference that. For more info on that issue see: How to Import SVG into GIS?

http://upload.wikimedia.org/wikipedia/commons/7/7e/N%C3%BCrnberg_Statistischer_Bezirke.svg