If you are using the standard form that we provide in the Contacts section of your account (Join My Mailing List), there is no way to modify the favicon for that page or the layout of the page. You can customize the colors and the required fields.
To get finer control, you would need to use our API to create a custom form. This method requires some web programming expertise.
Dave Berard Senior Product Manager, Constant Contact
View API documentation, code samples, get your API key.