GNU Free Documentation License I want to use this picture for one of my presentations. Do I have to mention the author there, and if yes, how do I mention him? It will be my first talk in English, so I don't really know how to correctly use/cite a picture from someone else.
As it says on the page you linked it, attribution is required. So, all you need to do is add the author's name under the image in your presentation.

The important thing here is that if you do not include the author's name, you are implying that you create the graphic and if you did not, then you should attribute it to the true author.