My temporary workaround is to edit thumbnailer.php in the theme:
LINE 201
/**
* Get thumbnails when post is displayed
*/
function wp_rp_get_img_tag($src, $alt, $size=null) {
if (!$size || !is_array($size)) {
$size = array(WP_RP_THUMBNAILS_WIDTH, WP_RP_THUMBNAILS_HEIGHT);
}
$size_attr = ($size[0] ? ('width="' . $size[0] . '" ') : '');
if ($size[1]) {
$size_attr .= 'height="' . $size[1] . '" ';
}
/* MY HACK */
//get image from ID if it's a number
if (is_numeric($src)):
$src = wp_get_attachment_image_src( (esc_attr($src)), 'thumbnail' );
else:
$src[0] = $src;
endif;
return '<img src="'. $src[0] . '" alt="' . esc_attr($alt) . '" '.$size_attr.'/>';
/* END HACK */
}