To change this, you will want to open the choosegallery.php page in the Admin folder of your PowerGallery and find the query that looks like this:
$result_gallery = mysql_query('
SELECT *
FROM pg1_gallery
ORDER BY id ASC');
Modify the ORDER BY clause to look like this:
$result_gallery = mysql_query('
SELECT *
FROM pg1_gallery
ORDER BY id DESC');