GIMP :: How To Make Object Look Like It Has More Sunlight On It

image editing in general, so I'm not sure what how to really say what mean because I don't know the correct terms (If there are any).

The most I can do to clarify what I mean is offer two examples. The first is a picture I found:

[URL]

The sunlight hits her head at the very top right of the image and transitions from white (direct sunlight) to brown (indirect sunlight) to black (almost no sunlight). Also, the light directly touches her hands and legs (white) and then transitions to indirect (pink).

The second example I can offer is on this very forum. On the header, if you look at the larger GIMP dog balloon (I think they're dogs, anyway...) you can see the white oval on top of the purple-ish nose. If you look at the top of the nose, right before it transitions to the black outline, you see an almost reflection-like effect.

Is there a function in GIMP to do this? Or, at least, some sort of work-around?