Hi Matthew,

You could take a look at the following Code Library for adding a watermark to the displayed images. However, note that you need to add the watermark before passing the images to the RadImageGallery:
http://www.telerik.com/support/code-library/adding-watermark-to-the-images-in-radimagegallery
 
With the provided solution in the Code Library, we are changing the "Status" of this feature request to "Completed".

You can place some div over the image, with your watermark, but it will not be visible when saving the image using the browser.
If we made such feature it will have the same result, which is not real watermark.