I've just tried it like this:

        optionsButton = new Button("_Options");
	optionsButton.setImage(new Image(StockID.PREFERENCES));
        helpButton = new Button(StockID.HELP);

The help button shows an icon & the text "Help"; the options button shows a "missing icon" icon and the text "Options". Even when I use the StockID.HELP for the options button it still shows the "missing icon" icon. I'm testing this on Linux.

I've now reported this as a bug https://github.com/gtkd-developers/GtkD/issues/300 including a tiny runnable example.