While in Virginia this past summer we visited Thomas Jefferson's summer home -
Poplar Forest. Of course I took tons of pictures (though we weren't allowed to take pictures inside the home!). Unfortunately the sky was a little bland on many of them. But, I have a file of skies on hand for just such occasions!
Before pictures:

I cut the sky out, slid the new sky underneath and combined the two pictures. And voila!
This is the final result:

Okay, so it's a bit more complicated than that. LOL. But that's the general idea. :)