If you upload an image that is available licence-wide, you can use this image from all of your emails. This means it’s an excellent solution for images you use often, like your logo.
You could theoretically use this method for all of your images, but we strongly discourage it. After a while, you will have so many images in your media library that you will lose all track of them. The folder system is also virtually impossible to restructure because the links in the emails you already sent won’t be correct anymore.
You can add images via control > media files. You can manage this media library however you see fit by using a folder structure you’re comfortable with.