Sending multimedia messages is not natively supported by Programmable Chat at this time. Until then, you can use the Message.Attributes property to pass a reference to the URL of an image stored on a media server along with metadata about the image (name, type, size, etc). See REST API message resource to find out more about Message.Attributes property. Base64 encoding of images and passing a string in the message will also work, but only for small images. A Message currently has a 400kb max size - and this is likely to be reduced further.