Post image with raw json?
You need to use the file resource to upload the file first; that will provide you with a fid (file ID), which gets attached to the field on the node. The payload for that request might look something like:

{
    "filename": "foo.jpg",
    "target_uri": "bar/foo.jpg",
    "filemime": "image/jpeg",
    "file": "iVBORw0KGgoAAAANSUhEUgAAAKQAAAFzBAMAAABC179RAAAAD",
    "filesize": "12345"
}


And then to attach it the field your existing code will change to something like:

"field_image":{
  "und": [{
    "fid": [FILE ID RECEIVED FROM FILE SERVICE HERE]
   }],
}