POST /api/alpha/upload/image - upload an image, get a url back. Use that to construct markdown/html as you wish. Uploaded images are associated with the current user and count towards their quota (50 MB by default, tweakable by instance admin)

GET /api/alpha/user/media - get all the images the current user has uploaded. Use this to provide a image picker.

POST /api/alpha/image/delete - provide the url of an image that the current user owns and it’ll be deleted.

Details in the swagger docs.