The setting "visual thumb selection" don't work by me in Firefox and Chromium.

Setting this option places thumbnails in the album thumbnail selection list (the dropdown list on each album’s edit page). In Firefox the dropdown shows the thumbs, but in IE and Safari only the names are displayed (even if the thumbs are loaded!).

Does that work for you?

In the source code i see the ```

It is true that the thumbs only show in certain browsers. This is a browser specific issue.
We will not work in this area in 1.x anymore as the thumb selection will be completely reworked in the already mentioned major release.