A few days ago we had a rare but welcomed snowfall. About 4 inches fell, it hasn't snowed like this for about 7 or 8 years where I live. Unfortunately I was at work all day and I knew the snow and light would have gone by the time I finished so I went out during my lunch break.
I took several photos but the light was very flat and all the images looked lifeless. So, I used the image you see below and simply changed the sky using Photoshop. I also removed some ugly man-hole covers using the clone brush.
Yep that's a much better sky, good idea and well replaced! I wouldn't have known if you didn't mention it. It's impressive that the lighting looks right on the snow even for a blue sky. Also the composition is great.