I managed to get it to work with this code (this will create the url for the thumbnail-image):
$a_id = get_post_meta($post->ID, ‘attachment_id’, true);
$size = ‘thumbnail’;
$image_src_array = wp_get_attachment_image_src($a_id, $size);
//get url – 0 – url; 1 and 2 are the x and y dimensions
$url = $image_src_array[0];
echo $url;
