Somehow the problem solved itself, at least to some extent. The format is portrait now though the black strip at the right is still there. I.e. help is still needed.
Simply posting the question on a support forum turne out to be enough for solving the problem. The homepage is now as I want it to be. No more help is needed, at least for now.
Sorry but it seems I’ve got the same pb http://naloudesbois.fr/ how did u you fixe it?
My problem was solved when I changed the thumbnail sizes as follows:
On the dashboard go to Settings > Media and adjust the thumbnail sizes to width 237 and height 302 with a tick next to “Adjust thumbnail to chosen sizes” (Or words to that effect: I use a Dutch version of WordPress). Finally save the changes. I had to remove and reload the images that I used for the messages though before the changes became visible.
Hopes this works for you as well. Good luck.
