Definitely extend the validator to a service provider . You can use an existing one app/Providers/AppServiceProvider.phpor create another one for verification.
Then in the method boot()add the following:
public function boot(){
$this->app['validator']->extend('min_image_size', function($attribute, $value, $parameters) {
$imageSize = getimagesize($value->getPathname());
return ($imageSize[0] >= $parameters[0] && $imageSize[1] >= $parameters[1]);
});
}
:
$this->app['validator']->extend('min_image_size', 'MyCustomValidator@validateMinImageSize');