I recently made a couple for president's day patterns using Gimp (a free download) and a few pretty simple tricks I read around here. The result is a shadow drawing rather than a line drawing, meaning the image is captured by the shadows in the picture rather than by lines like a drawing. It works best if the photo is NOT a direct face shot with a flash camera (there aren't many shadows to work with). The steps I use:
1. Paste the picture into Gimp
2. Crop the picture to the area you want to see (maybe just the collarbone up)
3. Image->Mode->Grayscale to change the mode to grayscale (rather than color)
4. White out the background around the face using a Gimp pen or brush
5. Color->Threshold then use the slider to get a good mix of black and white
6. Use a white pen or brush to get rid of black dots, and a black brush to fill in mixed areas
7. Do a black Fill Bucket to look for white islands that will fall out when you remove the black sections. (Control-Z will undo the black fill so you can see your picture again).
8. For tiny white island specs, just use a pen to blacken them.
9. For large white islands, look for a place to draw a white bridge to another white area (white is the wood left after cutting)
10. Zoom in and look for ragged edges that your saw can't follow anyway. You can change these to white or black to smooth the edge (Gimp doesn't have a Vector feature to do that for you).
11. Step back and see if you are leaving any fragile big white peninsulas. These would be large pieces of remaining wood with only a few weak bridges holding them to the rest of the board. Look for places to build a bridge that will not disturb the drawing but will provide some strength. For example, the part in Abe's hair was not in the original Wikipedia picture - it was added to strengthen the connection of the face to the rest of the board.
Anyway, those are the steps I use. It's all done in Gimp (free software) with personal or public domain pictures.
