1. Is it possible to show the Thumbnail View as default, and not the gallerie view at the galerie display?
In Admin edit categories set your default view as thumbnail
2. Is it possible to cut of the navigation of vbulletin (I use photopost pro with vbulletin integration)? I need this, because at the moment, the user must scroll to view the picture, and that is very usability unfriendly...
You would edit your header-inc.php and remove the variable $navbar
3. The Slideshow navigation shows me sometimes 4, than again 6 or 8 photos. Where can I set the number of max photos to show?
The slideshow navigation shows you all the photos in that cat you can view from the point you enter the slideshow to the end.
4. Is it possible to let guest only show 3-4 pictures, and then tell them to register? Or can I set, that guest can only view thumbnails?
No you can not do the first but in registration options in admin you can elect that users need to be registered to see images so they only see thumbs.
5. Where can I set the thumbnail width and heigh of the index Page photos for the categories, they are so small...
They are half the height of the normal thumbnail. You can not change them without modifying code.
6. The lightbox shows me the close Button in the same color as the background (white on white). How can I change this?
?? You sure you see the close button it is not white.
7. How can I tell photopost to order the albums on the index page, like the date. For example, not to integrate the newest album on the index page on the last position? Would make sense
Albums are not displayed on an index page only categories. Albums are displayed on the albums script by user.