Create file through services
If you need to create/update a file alongside the entity posted JSON you need to create the file in your code like:

//Implements of hook_services_request_preprocess_alter().
function my_module_services_request_preprocess_alter($controller, $args, $options) {

switch ($controller['callback']) {
case '_user_resource_create':
case '_user_resource_update':
  if (isset($args[1]['avatar'])) {
    $data = base64_decode(trim($args[1]['avatar']));
    $finfo = finfo_open();
    $mime_type = finfo_buffer($finfo, $data, FILEINFO_MIME_TYPE);
    finfo_close($finfo);
    // get file type png. jpg
    $ext = $mime_type ? str_replace('image/', '', $mime_type) : 'png';
    $file_name = str_replace(' ', '-', $args[1]['name']);

    $file = file_save_data($data, 'public://users/' . $file_name . '.' . $ext, FILE_EXISTS_REPLACE);
    $args[1]['field_field_user_picture'] = array(
      'fid' =&gt; $file-&gt;fid,
      'width' =&gt; 100,//set your own
      'height' =&gt; 100,//set your own
      'alt' =&gt; $args[1]['name'],//set your own
      'title' =&gt; $args[1]['name']//set your own
    );
  }
}


By that way you could create any image field with any entity easily.

From the best practice I advice you to use the module user picture field that let's you deal with the user picture as a field so you could have it in views with different formats instead of just have the file id.

Another tip is to send the image format as It won't be possible to get the mime type from the json decoded file source.

Sure you still be able to create a file through file services but I think you have to make the post request format www/url-encoded instead of json