The photos in your album are automatically sorted in ascending order by the date taken. You can change the order to descending.
To change the sorting order of the album displayed in the album viewer, click the arrow button on the photo library menu. Select either Sort Photos Ascending or Sort Photos Descending from the menu.
If you want to change the position of an individual photo, drag it to the desired position. A vertical line guides you as you drag the photo. To return to ascending/descending order (depending on the last default order selected), select Resort Album from the photo library menu.