If you want to display your content our an existing module in a customized image box like a lightbox and not a real message box from your system, then use Markit Fancy Popup. This module is created based on artviper script and will display your message in eye catchy fadein-fade out effect, and also has flexible configuration options that can be configured from Module settings page. You can define serveral parameters from width and height, show delay time, viewrs, etc.