function biotime_form_alter($form_id, &$form) {
  if ($form['#base'] == 'uc_product_add_to_cart_form') {
    $form['submit']['#theme'] = 'button';
    $form['submit']['#button_type'] = 'image';
    $form['submit']['#attributes'] = array(
      'class' => 'product-submit',
      'src' => 'images/add_to_cart.jpg',
      'alt' => t('Add to Cart'),
    );
  }
}