How to assign icons to a tree view node in a tree view....?

Depends on the environment: there are different controls for Web and WinForms for example.

For the Winforms TreeView, it's pretty simple: Set Icons for TreeView Control - Windows Forms | Microsoft Docs[^]

For Wed TreeViews, it's also simple - just set the ImageUrl property:

<asp:TreeNode ImageUrl="image.jpg"></asp:TreeNode>