I came to the result that for my purposes it is easier to use less and simplier code than paint.net.
If there is another Beginner in C# who is interested in it:
Here are some examples about simple drawing:
Simple Graphic programmin in C# Part 1:
http://www.developer.com/net/csharp/article.php/10918_1435391_2
Simple Graphic programmin in C# Part 2:
http://www.developer.com/net/csharp/art ... _1436621_3
But I am still interested, how to connect other applications to paint.net
Regards