I’m running into the same issue, except the image files aren’t attached whether Zip uploads is checked or not. It seems they are deleted before they can be attached, producing the following error in the Gravity Forms log:
2018-03-12 17:26:06.566267 – DEBUG –> GFCommon::send_email(): PHPMailer class returned an error message: Could not access file: /path/to/file/image.png