Shadow problem

[b60]

Well, at long last I'm finally getting back to this. I still don't understand the steps involved. What I'm doing is taking a rendered lawn with a so-so grass jpeg established and tree shadows cast on it. I then select this lit grass area in Gimp and paste in a more realistic grass jpeg. Obviously I can't paste this on top of the tree shadows because it would just cover them up, so I leave the shadowed areas unselected. But when the new grass is applied it makes the shadows look horrible, so I need a way to get the new grass into the shadowed areas.

I just tried going over this and I'm afraid I'm lost. Could you please explain (again!--sorry about this) your step-by-step process to solve the above problem?

I truly do thank you!