The image was converted to a format that create an appearances of depth using small circles. The size of the circles are different percentages in different areas to make the image darker or lighter. If you are familiar with stipple technique for shading that the best thing I can compare it to.
It is possible to do multiple colors when screen printing, but for the image of the girl only black ink was used. Check out the last picture I posted on the first page, you can see the detail of what I'm talking about.