Where do the pictures in the picker come from?

Picture Selection offers the pictures already in your book, your connected albums, and a search box for finding a specific one. Tabs for accounts you have not connected will have nothing to show — connect them first from My connections. Choose a picture and press Validate.

This applies to cover pictures. The photographs inside your book are placed and sized automatically and cannot be edited, cropped or filtered individually — to change what appears there, use Edit content.

Where to find it: Photo Editor → Change Image.