How to add product thumbnails to Checkout?
You can override the theme function uc_cart_review_table to accomplish this.

function YOUR_THEME_uc_cart_review_table($variables) {
  $items = $variables['items'];
  $show_subtotal = $variables['show_subtotal'];

  $subtotal = 0;

  // Set up table header.
  $header = array(
    array('data' =&gt; theme('uc_qty_label'), 'class' =&gt; array('qty')),
    array('data' =&gt; t('Products'), 'class' =&gt; array('products'), 'colspan' =&gt; 2),
    array('data' =&gt; t('Price'), 'class' =&gt; array('price')),
  );

  // Set up table rows.
  $display_items = uc_order_product_view_multiple($items);
  if (!empty($display_items['uc_order_product'])) {
    foreach (element_children($display_items['uc_order_product']) as $key) {
      $display_item = $display_items['uc_order_product'][$key];
      $image = uc_product_get_picture($display_item['#entity']-&gt;nid, 'uc_cart');

      $subtotal += $display_item['total']['#price'];
      $rows[] = array(
        array('data' =&gt; $display_item['qty'], 'class' =&gt; array('qty')),
        array('data' =&gt; $image, 'class' =&gt; array('image')),
        array('data' =&gt; $display_item['product'], 'class' =&gt; array('products')),
        array('data' =&gt; $display_item['total'], 'class' =&gt; array('price')),
      );
    }
  }

  // Add the subtotal as the final row.
  if ($show_subtotal) {
    $rows[] = array(
      'data' =&gt; array(
        // One cell
        array(
          'data' =&gt; array(
            '#theme' =&gt; 'uc_price',
            '#prefix' =&gt; '&lt;span id="subtotal-title"&gt;' . t('Subtotal:') . '&lt;/span&gt; ',
            '#price' =&gt; $subtotal,
          ),
          // Cell attributes
          'colspan' =&gt; 4,
          'class' =&gt; array('subtotal'),
        ),
      ),
      // Row attributes
      'class' =&gt; array('subtotal'),
    );
  }

  return theme('table', array('header' =&gt; $header, 'rows' =&gt; $rows, 'attributes' =&gt; array('class' =&gt; array('cart-review')), 'sticky' =&gt; FALSE));
}


With the code $image = uc_product_get_picture($display_item['#entity']-&gt;nid, 'uc_cart'); you can read out the product image and add it to the table.