Ok, now I have the solution!
In the Add Method of the TreeNodes you cannot specify only the index of the image of the node, but also the image of the selectedNode!
tn.Nodes.Add("name", "Node Text", 1, 1);
 :)

Because I have two different kinds of images, depending on the item itself.
And I combine the image directly with the new node when creating the tree.
The image is just red and green, and is symbolizising the "status" of the item.
And this has nothing to do with the automatical status "selected/unselected"....