Initially reported on: https://github.com/mautic/mautic/issues/1615
The image manager is not very easy to access. You have to go in the editor of the email builder or the landing page builder to have this ability.
Add an access to the image manager in the left-hand menu Assets with 2 sub-menus:
There, you’ll have the ability to upload and manage massively your images without the need to go in any editor.