Imagefields do not currently work with the new REST GET/POST/PATCH/PUT operations.
The plan is to let clients POST a base64 encoded string containing image data. A new deserializer for imagefields will decode that stream and save to temp directory. The deserializer will then return a tempfile uri.
For model code, see how devel_generate does it at http://drupalcode.org/project/devel.git/blob/refs/heads/8.x-1.x:/devel_g... That code does a little trickiness which you can ignore, by pre-creating 5 random images and picking from those.
When implementing this, also note that imagefields support additional properties like alt text (and title text?).