lpNewCplInfo is a variable of LPNEWCPLINFO, given as lParam2 parameter of
the CPlApplet function of you applet. The hIcon member is the Icon displayed
in the control panel for your applet. If you press this icon, your dialog
will be displayed.
Or do you wan't to display an Icon in your dialog and use it as button?
In that case, you can add an picture in your dialog (you can set the Type to
Icon in the porperties dialog) and wait for mouse click message. I belive
you can also add an button and set an icon as background image, see MSDN for
button messages to control buttons. Or you create an application-defined
window control see MSDN too.
To open a custom dialog use DialogBox() function