Firstly, it has to be said that it is impossible to protect images that are published in the world wide web. Even if the right click is disabled on a site it will still be possible to download an image by disabling JavaScript in the browser or copying it directly out of the browser's cache.
So, why this article?
While it is impossible to protect images that are publicly presented on a site there are many people who restrict the displaying of images to certain users or user groups by using Joomla!'s access levels. These parts of the site and the gallery will only be available for those users then, however if an unauthorised user knows or guesses the actual image paths and names (those ending with e.g. .jpg) he will be able to see the image anyway (that's because access checks can only be done within the PHP script and not for delivery of media files).
In JoomGallery, it is very easy to solve this problem: