I was using the Hueman theme and had to re-install wordpress. After that, I re-installed the theme too and all featured images of my articles are now shonw in a landscape format (so you cannot see the image in full size in the preview of the article when it is an image in high size). The latest article is also affected (so, the images look "cut"). Before that, all images were shonw in the right format (high or landscape) and you was able to see the full image in the preview/on the startpage.
I couldn't find any special option how to get the previous theme look.