-> if you're the owner of the image or an admin, deletes the image. returns basic success/faliure
images are stored in /images/uuid.png. Image::get_by_uuid(uuid) just gets the image from the folder, verifies that it is an image, and returns it. Image::get_by_username(username) gets all database images from the database and returns the uuids. the client is responsible for getting the images.