Tutorial Sky replacement using Paint.NET

Migrator's note: KR153 is the original author of this tutorial.

Attention, update! Before you start using this tutorial, read this alternative, made by kipate. It is a different way to the same result but it uses Photoshop. I suggest you read both of them through and decide for yourself. Thanks again , kipate. It´s always good to have alternatives. Download link below.
Here it goes: My first tutorial!
What you need:
-Your game screenshot (to find at C:\Dokumente und Einstellungen\Username\Eigene Dateien\Eigene Bilder\Focus Home Interactive\Cities XL 2012\screenshots in my case )
- Background shot of your choice. Better start a little collection first so you can find out which one fits best. *NOTE*: Both shots should be in a similar resolution (more or less) to prevent problems later!
- Paint.NET (many programs have similar functions but in this case I´ll use Paint.NET since it´s free and simple to use even for beginners)
Start Paint.NET and open your screenshot.

Choose the “Magic Wand” tool. (Now what this tool basically does is to select an area of pixels that lies in a similar colour area as the selected one. You can adjust the tolerance of this tool in the upper bar. Usually I choose a value between 20- 30 %.)
Click in the sky. You will see that the tool selected an area.

By holding Ctrl and clicking in other areas you can add the rest of the sky to the selected area. First try the bigger areas.

Now comes the tricky part. Zoom in:

You will notice the rest of the condense trail and that little fuzzy areas around some buildings. Add this to your selected area until the edges are as clean as possible. Remember to hold “Ctrl” when you select. Otherwise you start a new selection and have to start all over again!

After you are finished with the skyline zoom out and look for other areas to add.
As always I left the best for the end…

You see the fuzzy edges and that little areas between the cables? Yes, they got to go, too! This is makes the most effort but it also helps selling your picture a lot. (That explains why I din´t choose a picture with the ferris wheel, right?) And remember to hold “Ctrl” while you select!
And done:

Note: If you accidently click on an area that you didn´t wanted to add you don´t have to start all over again. Just hold the “Alt” button while deselecting the unwanted area. (You should see that the little “+” icon on your arrow changes to “-“. Or you can always get one whole step back by clicking "undo" in the upper section.
Now have a look at your selection all over again. Every pixel counts!
When you´re done click on Edit/ invert selection in the upper menu. Then click on Edit/ copy.
Open your background shot.

You can already see that the city will go beyond the lower end of this picture. So increase the canvas size at Image/ canvas size/ height. You can be generous since you can chop off what you don´t want at the end.

Click on Edit/ paste. The cut out city without sky should appear over your background shot. Move it with the “Move Selected Pixels” tool until the city blends into the landscape. Now you´ll see why it´s important that both pictures have a similar resolution.

When you think that it´s positioned correctly use the “Rectangle Select” tool and then click on Image/ Crop to selection and save it under a new name.
And that´s basically it. Of course there are still some flaws here and there and it surely doesn´t stand up to any professional´s work. I forgot to erase the airplane for example. You can use the “paintbrush” or “Clone Stamp” tool to erase little flaws like that. Just experiment with the tools to find out what works best for you.
The final shot:

I hope that this will be of help to some of you getting nicer pictures for your CJs or Desktop or whatever. If you have any questions or suggestions just comment or pm me.
Cheerio,
KR153
DOWNLOAD LINK Kipate´s Tutorial: http://sharesend.com/2rm0g5oa