// important the Type class
use Concrete\Core\File\Image\Thumbnail\Type\Type;

// check to see if the thumbnail type object exists before creating it
// - get the thumbnail type by handle "my_thumbnail"
$thumbnailType = Type::getByHandle('my_thumbnail');
// check to see if the the thumbnail type is an object or not
// - if it isn't an object, create a new thumbnail type "my_thumbnail"
if (!is_object($thumbnailType)) {
$type = new Type();
$type->setName('My Thumbnail');
$type->setHandle('my_thumbnail');
$type->setWidth(100);
// optional
//$type->setHeight(100);
// requireType() will create thumbnail types that cannot be deleted through the concrete5 interface
// $type->requireType();
$type->save();