Storing an deleting photos in albums

When you place a photo in an Album, the original photo is maintained in the Library. If you delete a photo from the Library it is also deleted from all albums that it is in. There is only one photo in storage, it is just displayed in multiple places. There is no option to delete a photo from the Library without it also being deleted from all albums.

Another take on Photos in album is the analogy that Photos' albums are like iTunes Playlists. A music track can be in multiple playlists but there is only one copy in the iTunes library. It's the same with albums in Photos.

The following is not to delete photos from the library and from albums leaving them in the library:

Deleting Photos from a Photos Library

1 - regardless of where you are in the library, i.e. Photos, Memories, Favorites, People, Recents, Imports, albums or smart albums, select the photo(s) you want to delete and use the key combination of Command+Delete to move the photos to the Recently Deleted smart album.    Without the Command key held down you'll be presented with a confirmation Yes/No window regarding the deletion. 

2 - click on the Recently Deleted smart albums in the sidebar.

NOTE 1: If you're using Photos 1.2 or earlier use the File  ➙ Show Recently Deleted menu option.

3 - in the Recently Deleted album you can opt to let the photo automatically delete after the 30 day waiting period is up, delete them immediately or restore them to the library.

NOTE 2: deleting a photo from an album, slideshow, book, etc., with the Delete key only deletes the photo from that item. There is no key combination that will delete a photo from a project and move it directly to the Recently Deleted smart album.

NOTE 3: deleting a photo from a Moment or Collection (the Photos window) or the All Photos window deletes ALL occurences of that photo in the library.

As FoxFifth says, this "album" business isn't always clear.

Pictures aren't exactly "in" an album. When you "put a picture into an album," its name is added to a list of pictures to display when the album is clicked. So two albums can both have the same picture name in their lists, and that picture will show up when you click either album. An album is a List Of Pictures. In an Album, when you click on a picture and drag its thumbnail to another album, you do not move the picture. You just add that picture's name to the new album's list.

So albums give a specific view of your pictures. My Grand Canyon album, for instance, offers a view of pictures taken on various trips to the Grand Canyon, and my Trip to California album shows the pictures on that trip, some of which are the same pictures as shown in the Grand Canyon album. 

The Library has all the pictures in it. Always. If you delete a picture from the Library, then it's gone from all albums. (Actually, it's moved to Recently Deleted for 30 days, and then it's gone for good.)

And when you "delete" a picture from an album, its name is removed from that album, but remains on the lists of other albums. Again, An album is a List Of Pictures. But if you use command-delete, then the picture is deleted from the Library and, so, removed from every album's list.