How to replace the sky with a Sunset
Step 1. I uploaded a nice sunset
Step 2. Used Edit -> Copy image to clipboard
Step 3. Upload my landscape picture.
Step 4. I used the Paint Bucket Tool
Step 5. Set the fill pattern to "your clipboard"
Step 6. Set the "Fuzz" to 20%, it may need to be
higher or lower for you.
Step 7. Clicked on center of sky for paint bucket.