Bugfix: remove floating part around image

This 'fixes' the problem of the image not showing up so it is an improvement. I'm not sure why the floating environment used to work (or still works) for the Dune 2.3 release. Building the dune website locally behaves strange at my machine (see comment in #74 (closed)) so I didn't investigate further.

If someone comes up with a real solution feel free to update this merge request (or close it) :)