currently clicking on a thumbnail will first open the mid-sized image and from ther the fullsize image. Is there a way to have the fullsize image clicking on the thumbnail?

Yes of course, but you will have to modify whatever theme you are using. For example on the Zenpage theme's `image.php` you can replace the `getImageURL()` with `getFullImageURL()` to open the full image directly. You can then attach some lightbox like that of the included colorbox plugin to that link to provide some kind of prev/next links.

Best read the theming tutorial to learn the basics of theming first:
http://www.zenphoto.org/news/theming-tutorial/