The asset library keeps your team's pictures together, each once, so a series can share its emblem and every book can take its cover art from one place. Open it from the menu under Assets.
Adding: upload a JPEG or PNG, and say what it is: cover art, an emblem (a PNG, so its background can stay clear), a map, an author photo or another picture. Give it a name, a line on what it shows for readers who can't see it, and its credit: who made it and on what terms ("Cover art by Ana Rey, licensed for this series"). A picture that's in the library already isn't added twice.
Using it on a book: on a book's page, the Cover card takes its artwork or emblem from the library, as the same file, not a copy. A book's own artwork or emblem can be kept in the library from there too, for the next book of the series. Each picture in the library lists the books whose cover uses it.
In the text: in the manuscript editor, Picture puts a picture from the library (a map, say) where your cursor is, with its description. When you save the version, the book takes its own copy, without what the camera or drawing program wrote into it (such as where a photo was taken), so the book keeps it whatever happens in the library. A picture larger than a book's text takes (30 MB, or 60 megapixels) isn't offered: keep a smaller copy in the library for the text.
Taking one out of the library leaves the books that use it as they are.
What you need: the Author plan. The library is your team's, so a Book Pass, which opens one book, doesn't include it.