<?= $this->Form->create($user, ['type' => 'file']) ?>
<?= $this->Form->input('image', ['type' => 'file']) ?>
<?= $this->Form->button(__('Save')) ?>
<?= $this->Form->end() ?>

public function edit($id = null)
{
    $user = $this->Users->get($id, ['contain' => ['Images']]);
    if ($this->request->is(['patch', 'post', 'put'])) {
        $data = $this->request->data;
        $user = $this->Users->patchEntity($user, $data);
        if ($data['image']['size'] > 0 && $data['image']['error'] == 0) {
        // Image is uploaded.
            if (!empty($user->image_id)) {
                // Delete old image.
                $old_image = $this->Users->Images->get($user->image_id);
                $this->Users->Images->delete($old_image);
            }
            // Trying to create a new Images entity with $data
            $new_image = $this->Users->Images->newEntity($data['image']);
            $new_image->filename = $data['image']['name'];
            $new_image->image = $data['image’];//This part seems to be incorrect.
            $this->Users->Images->save($new_image)) {
            $user->image = $new_image;
        } else {
            $user->image_id = null;
            $user->image = null;
        }
        if ($this->Users->save($user)) {
            this->Flash->success(__('The user profile has been saved.'));
            return $this->redirect(['action' => 'edit', $user->id]);
        }
        $this->Flash->error(__('The user could not be saved. Please, try again.'));
    }
    $this->set(compact('user'));
    $this->set('_serialize', ['user']);
}

{'image' => [
        'name' => 'image_name.png',
        'type' => 'image/png',
        'tmp_name' => '/tmp/phphmeViC',
        'error' => (int) 0,
        'size' => (int) 4003
    ]}