This snippet removes the action that inserts thumbnails to products in the loop and re-adds the function customized with our wrapper in it. It applies to all archives with products.

woo-loop-image-wrap.php

<?php
/* This snippet removes the action that inserts thumbnails to products in teh loop
 * and re-adds the function customized with our wrapper in it.
 * It applies to all archives with products.
 *
 * @original plugin: WooCommerce
 * @author of snippet: Brian Krogsard
 */

remove_action( 'woocommerce_before_shop_loop_item_title', 'woocommerce_template_loop_product_thumbnail', 10);
add_action( 'woocommerce_before_shop_loop_item_title', 'woocommerce_template_loop_product_thumbnail', 10);

/**
 * WooCommerce Loop Product Thumbs
 **/

 if ( ! function_exists( 'woocommerce_template_loop_product_thumbnail' ) ) {

	function woocommerce_template_loop_product_thumbnail() {
		echo woocommerce_get_product_thumbnail();
	} 
 }


/**
 * WooCommerce Product Thumbnail
 **/
 if ( ! function_exists( 'woocommerce_get_product_thumbnail' ) ) {
	
	function woocommerce_get_product_thumbnail( $size = 'shop_catalog', $placeholder_width = 0, $placeholder_height = 0  ) {
		global $post, $woocommerce;

		if ( ! $placeholder_width )
			$placeholder_width = $woocommerce->get_image_size( 'shop_catalog_image_width' );
		if ( ! $placeholder_height )
			$placeholder_height = $woocommerce->get_image_size( 'shop_catalog_image_height' );
			
			$output = '<div class="imagewrapper">';

			if ( has_post_thumbnail() ) {
				
				$output .= get_the_post_thumbnail( $post->ID, $size ); 
				
			} else {
			
				$output .= '<img src="'. woocommerce_placeholder_img_src() .'" alt="Placeholder" width="' . $placeholder_width . '" height="' . $placeholder_height . '" />';
			
			}
			
			$output .= '</div>';
			
			return $output;
	}
 }
?>