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.