The Media Manager in the builder has 3 tabs.

My Images - shows the images that you have uploaded from your computer as well as the free images that were downloaded.

Free Images - this has a list of free images from unsplash.com that you can download and use on your pages. The images will be optimized and compressed when they are downloaded using the media manager.

Upload - This tab provides the option to bulk upload images from your computer to use them on your page.