Currently, Memories on Facebook does not support being able to add photos when you view them.
From Facebook’s help page:
If there aren’t any memories listed, it may be because we don’t have anything to show for that particular day. You can check your Memories again the next day.
So if you had any photos for a Memory for the current day, you’ll see a photo for that day.
If you’d like a way to save photos to memories on your own terms, try out Little Memory, a private journal for self reflection and personal growth. You can add photos to your memories, and they’re yours to cherish in your own journey of personal growth. Memories on Little Memory are private by default, and you can share with those you trust or share anonymously.