You need to open the email with "Edit Draft", and then double click on the editable area. Once you see the editor, you can drag adn drop an image into the HTML section on the left.
Yes, landing pages are completely different. This has to do with the limitations of HTML in emails - most Email clients can only display very basic HTML.
On the landing pages, however, we can use drag and drop, because they are only viewed in web browsers - which are capable of displaying the HTML and CSS code it requires for dragging and dropping.