Well, on my webpage some one used the Klimt Hypatia(sic?) for the entrance page. Oddly enough, that's one piece that I really like and would like to use as a default icon. Especially as most of my lj posts are non-Smallville (though most of my comments in other ljs are Smallville related).

So, basically, female images. Preferably dark haired from Roman times, the 18th century, and/or Art Nouveau iconography and art.