By default, photos within an album are sorted by photo name, but you can change this if you want to. It's possible to sort your photos by "Name A-Z" or "Z-A", or "Date modified (oldest to newest)" or "newest to oldest", using the drop down list on the top left visible after selecting one of your albums from "My albums".
If none of the available options suit you, you can edit all photo names within an album, allowing you to modify the order your photos are shown. To do this, click "Edit album detail" which is in the top left in your photo album. From here you can edit all your photo names as well as change the descriptions and cover photo of the album.
The easiest way to order the photos is by adding numbers to the beginning of the photo name, in the order that you want the photos to show.
Remember that if you have more than 10 photos in the album you'll need to number the photos with extra zeroes. For example, add 001, then 002, 003, 004 etc. Then save your changes by clicking "Save changes".