Dear jean
There is easier software called Photobie which also has layers.This is much simpler learning curve. you may download the same from
Free Software Downloads and Software Reviews - Download.com.
Now your problem. select the portin of your image cut it paste it as a new layer. go tothe layer. adjust the lighting or the shadow as required.
merge the layer no 1 and layer which you have adjusted . Your Image is ready
if you stillhave any doubts please do not hesitate to ask
Yaksha