...at the album index you have a thumbnail with description. After clicking the image gallery is different and the thumbnail isn't in the post anymore...
Erm - yes it is :-)
My index for all available albums lists each with a heading, a photo, a description, and a note of the number of photos the album contains. This index is just a Category Template that I created to display my 'Photos' category.
If you click a photo or album name in the album index, you are taken to the post which contains thumbnails of all the album photos. The photo that was used in the album index is included, it's always the last one in the post and is the standard way WordPress works.
I just use several posts (one for each photo album). Each post has a gallery included in it. Each post is allocated to a 'Photos' category. There is a category template file for the 'Photos' category so that the list of albums looks different from a list of posts from any other category in the blog.
It doesn't get much more complicated than that for my albums.