First of all make sure the UV Set is good for a face. You should use cylindrical or spherical UV mapping, then optimize it manually by unfolding things (eg. ears are kind of fiddle.-work)
Then save the UV set as a template texture, with the triangles drawn, like UltimateUnwrap allows to do, or Lithunwrap. You may even have a tool that supports automatic optimized face unwrapping, instead of cyl or spere as basement.
I would then use this UV template texture as one layer of an image in photoshop or the-gimp. During painting works I'd set its transparency to 0.5, so it will shine trough and tell me where to draw.
First I'd try to find the eyes, the nose tip and the mouth. Just some dots. Tattoo may also be very useful for this step! You may also find it by try-and-error. Then I would take a photo and paste, rotate, scale and distort it until the eyes, nose tip and mouth fits the dots I made before.
Further optimations would have to be done. Finally I'd save this (without the UV Triangles) as PNG, JPG or DDS.
This is something I did this way (tho, it was already unwrapped):