When you edit the form in Design Studio, Form Settings - Form Theme adds a gear on top right corner, just under the Finish button. Clicking the gear allows to set a custom CSS for that from.
Images and JavaScript are specific to landing/HTML pages. That is not in the context of forms, Marketo or any other. By default a form is an object defining input fields. Other elements are defined in the page itself.
W3C's definition
An HTML form is a section of a document containing normal content, markup, special elements called controls (checkboxes, radio buttons, menus, etc.), and labels on those controls. Users generally "complete" a form by modifying its controls (entering text, selecting menu items, etc.), before submitting the form to an agent for processing (e.g., to a Web server, to a mail server, etc.)
Source:
http://www.w3.org/TR/html401/interact/forms.html