Parent and Child classes

Another way to approach this is to focus on making the TreeView in Form1 available to class1. Since Form1 creates class1, this is easy.

In class1 add a public Property of Type TreeView:

public TreeView ParentTreeView { get; set; }

At the point in your code where Form1 has instantiated class1: insert the reference to the TreeView into class1:

MyClass1 = new class1();
MyClass1.ParentTreeView = Form1.F1TreeView;

Now, your instance of class1 can access Form1's TreeView directly.

If there are, potentially, many subscribers/consumers to/of an Event, consider having the Class raise an Event.

When there are, potentially, many instances of a Class that need access to an Object, consider insertion of an instance of an object (in this case a TreeView).

In this specific case: changing the ImageIndex of a TreeView doesn't require any other external information. If whatever class1 needed to interact with in its parent required many other references, then, of course, using an Event/Subscriber model would be more appropriate.