To keep your premium images safe you might have to consider to add watermark to images in WordPress. In WordPress you can do this by installing one simple plugin Easy Watermark. It allows to add watermark to all images automatically on upload to WordPress media library or you can do it to existing images manually. Manually you can add one single watermark all at once too. The format of watermark could be an image, text or both.
Here are the cool features of this plugin:
First of all get the plugin and install it. Upon installation you will get Easy Watermark under Settings menu. There are 3 options available General, Image and Text. Set the parameter according to your requirements:
To add watermark to existing images go to Media->Easy Watermark menu:
And this is the Easy Watermark metabox on attachment page :