You can use the class described in the link below:
Tooltip with Images
Then, supposing our Treeview is called treeView1, use the NodeMouseHover event like so:
private void treeView1_NodeMouseHover(object sender, TreeNodeMouseHoverEventArgs e)
{
ToolTipWithImage t = new ToolTipWithImage();
t.Text = "Hello";
t.ImageFile = @"C:\aRandomImage.jpg";
t.SetToolTip(treeView1);
}