get a picture of sky then select what you want to keep and copy it over to the sky picture or you can use a mask and paint out the sky
This is the best sky replacemet tutorial ever...especially where trees are concerned
http://www.skeller.ch/ps/easy_sky_replacement.php
Dave


Had a go at a shot of Blenheim Palace, Method was basically google a picture of the sky, download, open photo's, create 2 layers of the background photo block layer 1 and magnetic lasso sky of layer 2, delete sky, deselect lasso area, drag sky scene onto the layer pallet of background image, reselect lasso area, invert selection, delete excess, view all layers and edit if necessary, flatten image, done.
Before
Blenheim Before by Natty94.2, on Flickr
After
Blenheim After by Natty94.2, on Flickr