Consolidating two websites into one, but migrated images not appearing in Image Library

By default, when you upload an image via the admin, it’s metadata is added to the database, and that’s how you can search through them in the media library etc. If you upload images via ftp then they won’t appear in the DB so won’t show up in the admin area.

Use a plugin such as https://wordpress.org/plugins/media-from-ftp/ which will let you loop through every image and add them to the DB if they don’t exist already.