How do I make the context menu appear only when clicked at certain portions of the Control?

You can listen to the Popup event, determine where the mouse was clicked and selectively make the menu items visible in the menu as follows:

		// In C#
		private void contextMenu1_Popup(object sender, System.EventArgs e)
		{
			// Get current mouse click position in the control (assuming pictureBox1 is the control):
			Point ptClick = this.pictureBox1.PointToClient(Control.MousePosition);
			// Get the rectangle where you want to show the context menu.
			Rectangle preferredClickRect = new Rectangle(0, 0, 50, 50);
			if(preferredClickRect.Contains(ptClick))
			{
				// Show all the menu items so that the menu will appear
				foreach(MenuItem item in this.contextMenu1.MenuItems)
					item.Visible = true;
			}
			else
			{
				// Hide all the menu items so that the menu will not appear
				foreach(MenuItem item in this.contextMenu1.MenuItems)
					item.Visible = false;
			}
		}

		’ In VB.Net
		Private  Sub contextMenu1_Popup(ByVal sender As Object, ByVal e As System.EventArgs)
			’ Get current mouse click position in the control (assuming pictureBox1 is the control):
			Dim ptClick As Point =  Me.pictureBox1.PointToClient(Control.MousePosition) 
			’ Get the rectangle where you want to show the context menu.
			Dim preferredClickRect As Rectangle =  New Rectangle(0,0,50,50) 
			If preferredClickRect.Contains(ptClick) Then
				’ Show all the menu items so that the menu will appear
				Dim item As MenuItem
				For Each item In Me.contextMenu1.MenuItems
					item.Visible = True
				Next
			Else 
				’ Hide all the menu items so that the menu will not appear
				Dim item As MenuItem
				For Each item In Me.contextMenu1.MenuItems
					item.Visible = False
				Next
			End If
		End Sub