The best way as +smooth+ said is to buy a high res. image and modify that.
You can also
- - Resize the image to its final version.
- Then use filters such as Noise>Median.. and try to get rid of the big pixels as much as you can.
- This way you will lose the details but keep the colors and shapes
- Then divide it to small parts, which you want to add details
- Now use similar available sky photosfor each part and add detail to each small part
And I think it might take you a day or two …