adding photos to album
How to add photos to an existing album
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Method 1: If you have already imported the pictures to Photos, then they appear at the bottom of the Library window and also in the Imports window. You can select them there and drag them into the album. Keep in mind that albums always show copies, so don't expect the pictures to disappear from Library or Imports when you drag them to the album.
Method 2: If the pictures are in a Finder window on you Mac, you can select them there and drag them to an open Album in Photos. They will also show up in Library and in Imports.
Method 3: If the pictures are already in another album, you can select them and drag them to another album. Again, albums show copies, so the pictures will still appear in the first album. If you don't want them there, then select them and hit delete. They will continue to exist in the new album, in Library, and in Imports.