All images that are uploaded to your account are processed and stored for re-use.
You can access uploaded images a few different ways such as within Marketing Messages or Your Website then by clicking into Images & PDFs.
The Media Manager will show you all of the uploaded images, you can click into each image to edit ALT & Title Attributes as well as Delete any unused images.
Note: If an image is being used by the web builder, you won't be able to delete it until it's been removed. This is to avoid accidentally deleting an import image. However, images in the blog or other content may still be deletable.