Helper snippet to render images with an image style in drupal 8. Read more about Render arrays in drupal 8 here.
$imageUri = isset($node->get('field_image')->entity) ? $node->get('field_image')->entity->getFileUri() : null;
if($imageUri) {
$image = [
'#theme' => 'image_style',
'#style_name' => 'medium',
'#uri' => $imageUri
];
$markup = render($image)
}