I found a workaround for my problem. Changing the SubMediaType to TV-Show (keeping the MediaType as Picture) and filling the Series-Tag with anything seems to force MC to display only the first picture. I added a IsCover tag that is 1 if an image is a cover. Sorting the albums according to IsCover(z-a) always puts the cover on the first place, thus displaying it as cover picture.
It is not necessary to create a view that categorize the pictures by the Series-Tag , the default Album view works fine.
However this workaround works only if all elements in a category have the same series tag. So if you want to use a statict cover for every album, than the series tag should not change inside any album.
I hope we will have native support for covers for arbitrary fields in MC20, as this solution introduces additional work for the user.