If it's only high resolution images, it'd likely be the $config['maxImageResizePixelCount'] setting. This defaults to ~20 megapixels; anything above that won't have any image processing done to it.
You can adjust this in config.php by setting:
Code:
$config['maxImageResizePixelCount'] = 25000000;
(or whatever value is appropriate, with ~1 million being 1 megapixel).
Note that larger images require more memory to process. You may need to increase your PHP memory_limit to avoid errors.