what the documentation is saying is that the thumbnail link can be downloaded by right clicking on the thumbnail.
if you want to be able to download the large image, you need to read further and define the link mapper function:
If it's not enough for you and you want the visitors to be able to save the image from inside Slimbox, you can modify the "autoload code block" so that a link to the image is added automatically after the image caption. This is achieved by defining a linkMapper function... 
