Here are some progress screenshots of my airbrush painting of model Adriana Lima. I made this for a curious friend who just started using Photoshop.

Finished painting: [link]
Original photo: [link]

Technical: Photoshop 6.0. Wacom. Airbrush (hard edge brushes) and eraser only. Working size is 600x600. No tracing, paintovers, or filters.

Steps:
1. Sketch: Drawing from photo-ref with no tracing. 1pixel brush, 100% opacity in a single layer.
2. Base colors: Base for skin, hair, shirt, and chair each in a separate layer underneath the sketch layer. Darkest shadows added in black in another layer.
3. Basic shading: Working from darkest to light, in a layer under the sketch.
4. Basic hilights:
5. Turning off the sketch layer
6. Facial features: adding color to lips, eyes, misc. adjustements to nose
7. Facial features 2: adjusting shading on nose, adding higlights to lips
8. Clothes: cleaning edges on shirt, detail on strap, necklace and lacy texture on front of shirt
9. Background: adding base colors for background, texture on chair
10. Background detail: chair back, wall moulding, chair pattern
11. Hair: adding tendrils near face, hightlights, flyaways
12. Misc: adjusting details forehead, back, hair, arm, chair