There is no guarantee to be given to any of your photo which is uploaded to your website. It can be indexed by search engines and thus available for anyone through respective search terms. You have to set watermarks to your pics, somehow to prevent the usage of such pics.
Here is a website: http://picmarkr.com
They are providing quite nice option of getting watermarks in your pics without making it ugly and of course at free of cost. Though, there are several internet photo copy experts who can photoshop your pic and remove the water mark straight away.
Hope it helps a bit