$productId = ‘SOME_PRODUCT_ID’;
$product = Mage::getModel(‘catalog/product’)->load($productId);
$productMediaConfig = Mage::getModel(‘catalog/product_media_config’);
 
$baseImageUrl = $productMediaConfig->getMediaUrl($product->getImage());
$smallImageUrl = $productMediaConfig->getMediaUrl($product->getSmallImage());
$thumbnailUrl = $productMediaConfig->getMediaUrl($product->getThumbnail());

more info:
http://www.anyknowledge.com/magento-get-original-image-url-of-a-product/
http://www.magentocommerce.com/boards/viewthread/69911/