If you have an image style specified in Drupal 7 and you'd like to programmatically render an image thumb nail with that image style, try something like this:
$image = theme(
'path' => 'public://my_example_image.jpg',
'style_name' => 'my_custom_image_style',
'alt' => 'Alternate Text',
'title' => 'Title Text',
This would work if you had an image style specified at admin/config/media/image-styles that was called 'my_custom_image_style'.