Facebook Connect module
A quick look at the code shows that this is being created via:

function theme_fbconnect_login_button($text, $attr) {
  return '&lt;fb:login-button ' . drupal_attributes($attr) . '&gt;' . $text . '&lt;/fb:login-button&gt;';
}


which shows that Facebook is creating the button, and thus suggesting you are stuck with what you got.  Now, there may be some hidden attribute incantation to change the button, but given how brand conscious Facebook is, I doubt they will allow you to change it up using their SDK.