Add an <Icon> element to the page (Via right-click>Add Element).
Then drag an image from Finder/Explorer onto the page element.
If you next check the properties (Window>Show View>Properties) of the page, you'll note an html.icon property referring to the image file, which has been copied to the project's web/images folder (viewable via Window>Show View>Navigator).
Regarding the <Icon> element's value property, note that it is only editable for Constants.
For best results, use the Firefox browser..