The problem with basing bump maps off a desaturated texture (you might want to change the mode from RGB to Greyscale to make the file size smaller by the way) is that the dark and light parts of the texture usually do not correlate to the high and low features of the skin.

For instance, say the texture map shows a mole or beauty mark on the figure's face.  If you take that into Photoshop and desaturate it you'll discover the problem when you use it as a bump map.  In bump maps, white is high and black is low, right?  That dark mole or beauty mark is going to create a pit on the figure's face.  If there are veins visible on the texture, they are going to create shallow grooves on the figure.  Wrinkles are going to become cracks and fissures.  Shadows that are part of the texture are going to create depressions.  And if there are any highlights that are part of the texture they are going to cause bumps on the figure because they are white.

It's ok to use a texture as a bump map if you need to give an object like wood or stone a little bit of texture provided that texture is relatively even.  But even then you might have problems.  If you took a brick wall and made a bump map out of the texture, the mortar between the bricks would stick out because the mortar in the bump map is white.