NGClient generates in the index.html 2 link tags for icons:
By default those are the servoy icons, these are places in the webapp root dir, that is in a developer/application_server install:
you can replace the files there with the right sizes. When creating a WAR file for deployement servoy will take those files from that developer installation and include it in the WAR.
For max compatibility also place a favicon.ico file there besides those 2 above. Some browsers will auto look for that one.
As an example Chrome will use the smaller icon for the icon to display in the tab and if you create a (desktop) shortcut it will use the larger one.
NOTE: the files where called favicon75x75.png and favicon200x200.png before 8.1.2