Based on this forum.

Web designer (of design of website itself) sometime needed a little bit more than CSS theming of Drupal buttons - change it in image in example. This can be done in 'template.php' file in your theme folder. First you need to define image buton itself - so add this code:

function phptemplate_button($element) {
  $return_value = theme_button($element);
  if ($element['#button_type'] == 'image') {
    $return_value = str_replace('type="submit"','type="image"',$return_value);
  }
  return $return_value;
}

At second - change the button of webform. Add:

function NAME_OF_YOUR_THEME_webform_form($form) {

    $form['submit']['#theme'] = 'button';
    $form['submit']['#button_type'] = 'image';
    $form['submit']['#attributes'] = array(
        'src' => 'NEW_BUTTON_IMAGE.png',
        'alt' => t('Send'),
        'title' => t('Send')
        );

  return drupal_render($form);

}

Your button is going to live... Clear all caches and registry of theme!