How can I change the icon that appears in the titlebar during the installation?

You can specify a 48x48 GIF or PNG image to be shown in the window manager task bar for Unix systems using the project property. This will work for most window managers. The following example will set the titlebar icon:

<project>
   ...
   <wmImage>/path/to/titlebar/image.png</wmImage>
   ...
</project>