Add style class for submit button to contact-form stylesheet.css in wp-content\plugins\contact-form-7\stylesheet.css – e.g.
.contactSubmit {
border:none;
width:238px;
height:30px;
background-image:url(images/submit.gif);
background-repeat:no-repeat;
}
Add submit.gif to /wp-content/plugins/contact-form-7/images/
Define submit button in form:
<p>[submit class:contactSubmit ” “]</p>
Contact Form 7 updates will lose the css and image file – need to redo after updates.