After you create a project in Photos, like a book or a slideshow, you can add more photos to it with a simple drag and drop from the sidebar.
First, make sure the sidebar is visible on the left-hand side of Photos. You can do this in two ways:
- From the View menu, choose Show Sidebar.
- Or, Press Command-Option-S.
With the sidebar visible, you can see all your albums and projects. Here's how to add photos to a project:
- Select an album in the sidebar. You can select one that you've created, or you can use the All Photos album.
- Select the photos that you want to add to a project. To select multiple photos, hold down the Command key as you click.
- Drag the selected photos to the sidebar and drop them on the project you want to add them to.