I've had this imaginary girl stuck in my head for a week, like an addictive song . So I said to her "you don't belong in here, dear. Please get out of my head and onto my screen." Apparently, she was reluctant, because it took a lot of effort to get her on the page.
I used Opencanvas to sketch this and to set up the basic lighting and color scheme. And then I had some trouble figuring out how the light would hit near her eyes, so I looked up some b&w photos of Christina Aguilera and Adriana Lima to help with that area. I used Painter 7 to refine the skintones and Photoshop for most of the detailing.