As this question as I have understood, interests many, I would like to tell about it separately. ( In advance I am sorry for my bad English
)
There are some ways, but they, as far as I know, not strongly differ from each other.
I do it so.
1.To take a photo with the high permission. (Unfortunately, I very badly draw what not to use a photo, for this purpose.)
2.Picture is drawn on its basis.
3. Then it is drawing it is processed in a Photoshop CS5. And, of course, to put imagination that the result would turn out beautiful.
It is all.
The main complexity - correctly to choose a photo because there are many nuances.