I have the following problem:
In the gallery
(www.thw-jugend-frankenberg.de/Bilder2013/Landesjugendlager/Ausflüge/)
are images uploaded with a dimension of 600 x 902.
The images are displayed correctly in the gallery, only the thumbnails are rotated by 90 degrees.
How can I rotate the thumbnails without having to rotate the images?

That will depend on several things. You should go to the image page in admin and see if there is a rotation option. If so, you can set the rotation there.

If not, then your graphics library does not support rotation and you will have to correctly rotate your images before uploading them.

thanks, the problem is solved

Now a new problem has shown:
www.foto-hoerl.de/galerie/babybauch/
on the page the last image.
This image is black / white in the original and has been also uploaded.
Only the thumbnail has color.
Clearing the cache did not change image
So there are many images that have been uploaded actually worked, but the original images are displayed.

Problem is solved

The link does not work. But if you are saying the original image is b/w, then the thumb cannot be color magically at all.

There are also a theme options to set the image/thumbnail to gray-scale. Be sure you have not selected that.