it have on Photoshop, not sure about GIMP.
Edit>Transform>Wrap
some guide lines will appear, me the center smaller, fixing the size of both sides at the same proportion

Hmm, I'll try it, thankd

update :P
I'm gonna fix the seam in the back and change the color of the eyes maybe

You have made it DAMN blurry. you need to have some sharp edges. otherwise it sux.

But my gosh i do love those teeth.

the eye colour look good, the based colour too, now you need to add the details, and make it more sharp like aerox said
plz fix the back

I think the shading could have been done much better, not saying it is horrible, but there was so much potential in that great sketch.

In fact I think this version is the best one yet.
http://i1220.photobucket.com/albums/...ntitled-11.gif

So I don't think the thing you should worry about is the color of the eyes, I think your focus should be on either re-doing the whole color/shading process or scrap it and go with the head above.

boom
bam
pow

no variation of line weights, blunt pencil
hands look like claws
the girl looks ok, fat chin is the main issue
the clothing folds are nice, but anatomy go

i haven't begun my anatomy studying yet so no worries

what do you mean by blunt pencil

the girl, was drawn with a blunt pencil in places, didnt sharpen the pencil often enough, really thick lines
and generally you use quite thick outlines on all your works