Image as form submit
You could try a modification of this: found it here.

&lt;?php
function theme_form_comment_form_alter(&amp;$form, &amp;$form_state, &amp;$form_id) {

  $form['actions']['submit']['#type'] = 'image_button';
  $form['actions']['submit']['#src'] = base_path() . path_to_theme() . '/img/button-off.gif';
  $form['actions']['submit']['#attributes']['title'] = t('Create Comment');
  $form['actions']['submit']['#attributes']['onmouseover'] = "this.src='" . base_path() . path_to_theme() . "/img/button-on.gif'";
  $form['actions']['submit']['#attributes']['onmouseout'] = "this.src='" . base_path() . path_to_theme() . "/img/button-off.gif'";

}
?&gt;