Thanks, fivecat. 

That has partially answered my question, but how can you create an accurate alpha map of the brows, i.e. nice and sharp without feathering around the edges ? .... I have tried something similar in the past but because of indistinct edges, the eyebrows end up looking like they have been colored out of a bottle.

I should add that the paint programs at my disposal are GIMP v2.1.13, Ulead Photompact v10 and Adobe Photoshop Elements v2