In ArcGIS Online, go to Content>My Organization and then in the left-hand side column go to Item Type>Files>Images. This will display an overview of all of the images in the organization and provides the option to categorize or delete multiple images at one time. If there are a lot of images in the organization, click Items Per Page in the bottom right-hand corner of the page to show up to 60 or 100 images at a time. Unfortunately, there isn't a way to see whether or not an image is currently being used in a project so that may require some extra detective work amongst colleagues to find out that information. Hopefully this at least helps you get a start in reviewing all of the images in the organization and making some headway in removing unused images.