how to add an image to a cell in cellTable in GWT

I want to add an image in a cell in the CellTable. After reading the documentation, this is what I did,

Column<Contact, String> imageColumn = new Column<Contact, String>(new ImageCell()) {
    @Override
    public String getValue(Contact object) {
        return "contact.jpg";
    }
  };
table.addColumn(imageColumn, "");

Well, there's an empty column in the table now and no image in it. Am I doing something wrong here? Any suggestion is appreciated. Thank you.