To add photos to a Series book, login to the Chatbooks App and tap into your latest Series Volume and follow these steps:
Tap the + sign in the bottom right-hand corner
Tap "Add Photos"
This will automatically open your Camera Roll. Tap into the photos you want to include in your Series Book.
Once you've selected the photos to add, tap "Continue with X photos" in the bottom of the screen.
Leave the Chatbooks app open while those photos download to your series book.
Your photos are now added!
Here's a video how-to following this process as well: