Technically and aesthetically the original image has a number of flaws: The warm colours and severe highlights suggest that the original image was shot in daylight and then masked
against a white background. The masking has been
done freehand and you can see that there are
jagged edges and missing elements. The focus is
a little soft too and the colours are biased toward
red. Jpeg compression artifacts are subtle, but
the resolution is quite low. None of these flaws
represent a problem however when it comes to
applying this illustration/posterising effect as
you are no longer dealing with a continuous
tone image and the colours are of your own
choosing. Focus problems are also
unimportant as you are defining your own
edges.

The method of creating this style of
illustration is not far removed from the
techniques used in the creation of basic
colourised black & white photographs.
You create colour layers above the
original image in the same way that you
would for colourization, only in this case
you then change the blend mode to normal
thus leaving yourself with a very minimalist
version of your photograph. There are two
methods of selecting your coloured areas.
You can use a freehand approach and
simply paint solid colour layers over the
original image, or you could use bezier
curves to create a crisper, more genuinely
stylised version of the photograph. The
level of detail you chose to include is a
matter of personal preference and will vary
according to the nature of the image.