My code is like this, why i see the ImageIndex as well.
Once i click the checkbox, I like to display only the StateImage 

And also, if i use 
  ht.Contains(htOnStateIcon)
Never hit this.

ps. If i use ImageIndex instead of StateIndex, it shows the single icon but
it's not reflect the change properly.

__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{

    AnsiString s;
    TTreeNode *pNode = NULL;
    TTreeNode *pAddToNode = NULL;
    TTreeNode *pChildNode;
    for ( int i = 0; i < 20; i++ )
    {
        s = AnsiString( "test node") + AnsiString(i);
        pNode = tv->Items->AddObject( NULL,s.c_str() , (TObject *)0xffff );
        pNode->StateIndex = 2;//checked icon..

        pAddToNode = pNode;
        for( int j=0;j<3 ;j++ )
        {
            pNode = tv->Items->AddChildObject( pAddToNode ,s.c_str() , (TObject *)0xffff );
            pNode->StateIndex = 2;

        }
    }
}

void __fastcall TForm1::tvMouseDown(TObject *Sender, TMouseButton Button,
      TShiftState Shift, int X, int Y)
{
  if ( Button == mbLeft )
    {
        TTreeNode *pNode   = ((TTreeView*)Sender)->GetNodeAt(X, Y);
        if( pNode )
        {

            THitTests ht = tv->GetHitTestInfoAt( X, Y );
            if ( ht.Contains(htOnStateIcon) || ht.Contains(htOnIcon))
            {
                tv->Selected = pNode;
                if ( pNode->StateIndex == 0 || pNode->StateIndex == 2 )
                    pNode->StateIndex = 1;
                else
                    pNode->StateIndex = 2;
            }
        }
    }
}