There are several ways to add an asset to the library. For example, whenever you use the Media button on the toolbar to insert an image, it will also appear in the library.
When you add a file to a project's library, a copy of that file is made within the Captivate project's file. If you have a lot of assets, it can make a project's file size pretty big. Removing assets that you're not using will help keep the project's file size as small as possible.
Click the Select Unused Items button in the Library panel.