Step-By-Step Instructions:
1. Open a photograph that has a sky background in Photoshop. Make sure the photograph is on a separate layer (not the background layer).
2. Click the "Edit in Quick Mask Mode" button.
3. Select the Paint Brush and choose black as your color.
4. Paint over the sky with the paint brush. When in Quick Mask Mode, the paint will be transparent red.
5. When you are done, click "Edit in Standard Mode" button.
6. Goto Select > Inverse. The entire sky should now be selected.
7. Click the delete key to remove the sky background.
8. Click CTRL+D to deselect.
9. Create a new layer underneath the photograph (inbetween the white background layer and the top layer). Paste your new sky background image on this layer.
9. Select the Eraser tool, zoom in and erase any signs of the old background.
