Before you start a Picture book there are a few things you can do to mix the choice of art up, but once you start a book you can't change or add to your art choices.
When you click the 'create' tab you can either choose art from a single artist, or search for an art 'tag', which will find art on a certain subject - but from many artists.
If you would like to create a book using a tag then click on 'create' then take look just above the top picture on your right - you'll see a 'search all artwork' bubble.
If there is a specific word you are thinking of (like animal, or blue, or happy) type it into the search bubble. This will bring up all the artwork associated with that tag. You can create a story using this art, instead of using art only from one artist.
Once you have your search results click on the 'use this artwork' button above the top left picture to create a story using the tagged art.
As a tip, you can search for more than one tag - 'cat, blue' for example - these searches are subtractive though, and not additive. If you search for 'cat, blue' you will get pictures that contain only both those tags, not all pictures with 'cats' and all pictures with 'blue'.
If you search for an artwork tag, then click on one of the pictures you'll be taken to the artist's gallery. If you then click 'use this art' while you're in the gallery your storymaker will use the art from the artist, rather than from your tag word search.
For your Longform books you can use a different artist or tag for each of your chapters. Chapters are individual longform 'books', and once you finish writing them, you can join them together.
When the first chapter of a longform book is published there is a link underneath to begin the next chapter, and this makes the link automatically.
If you haven't published the first chapter yet, or wish to use different art, you can write the next chapter as a separate book then join them later. You can use a different artist for each chapter for the interior art this way, but they will all share the cover image of Chapter 1.