admin/config/media/image-styles

$style='full_content_width';
 $path=$node->my_img_field['und']['0']['uri'];
 $style_url = image_style_url($style, $path);
  print "<img src=".file_create_url($style_url)." >";

drupal image:styles:flush image_style

drupal image:styles:flush image_style