You can drag images onto the page to upload them or you can use the upload button inside the file manager. You can also create folders. Note that when dragging images on the canvas they’re uploaded to the root folder of your magazine.

Deleting a file can sometimes be dangerous. The world won’t end but when the file is used somewhere in the magazine or in other duplicated magazines it will be gone in all those places, so be careful.
Uploading or creating folders can be found under the "menu" button in the top left of the file library screen.