Photo order in Photos

In the concerned folder, go to View tab and click and expand view ribbon. In the View file ribbon, you can go to Arrange list for arranging the pictures as per your requirement. It will give you an option for arranging them by Date, Person, Type, Name, Rating and etc. Keep posted if you require further assistance. 

[Link Edited by Moderator]

For photos in an album, you can go to View>Sort and have them arranged by title or date. You can also drag pictures around to put them in any order you like. But if you later choose title or date, your custom order will be unrecoverable. But if you want more than one ordering, you can make a second album with the same pictures in it, and they they can be ordered in that extra album as you like without affecting the first album's order. If you want to preserve a custom order, you can add titles to the pictures with numbers in the order you want, and then sort them by title. Keep in mind, the titles belong to the picture, not the album-- you can't have different titles in different albums.

Here's the photo sort ordering in the various views in Photos:

You have to put them in an album. All the other views have there own ordering method.

We are talking about Apple Photos here, not Google Photos.