This is done with:
<link rel="shortcut icon" type="image/x-icon" href="/images/icons/favicon.ico" />within the HEAD element of the document. in the example here, "favicon.ico" should be a small rendition of the page logo. For ZUG, try using just a single penguin.
With this, the designated image will appear before the URL on most browser address fields, on the tabs in Mozilla, in Favorites lists, etc.