There are many ways you can organize your gallery photos.
Rearranging Photos
The photos in your galleries are displayed in alphanumeric order by default.
To sort alphabetically, go to Sort by > Filename A-Z or Filename Z-A.
To sort photos by timestamp, go to Sort by > Timestamp Old-New or Timestamp New-Old
To sort photos in a custom order, drag and drop the photos into the order you'd like. If the photo has a blue frame, that means it's selected. (Pro-tip: to sort photos in batches select the first photo and hold down the Shift key while selecting the last photo. All of the photos in-between will be selected.)
Use Albums
You can sort your photos into Albums, which are like folders or groups of photos within a gallery. For example, if the "Smith Wedding" is your gallery, you might have albums for "Ceremony," "Reception," etc. For a sporting event, a photographer might set up albums for each team, game, and/or day of a tournament.
Learn more about using Albums and their benefits here: Using Albums