Hello, my theme has some issues related to vertical image in theme check.
I don’t want it to get cropped (and reviewers don’t allow this, from what I know), but in single.php templates this image gets really tall, because I need images to be 100% of container’s width, because of a section which is over the image and has a gradient background.
To have a better understanding, please see https://torch.dp-themes.com/utest/2012/03/15/template-featured-image-vertical/
So, how theme devs deal with vertical featured images ?
(now I set width: auto, you can inspect and give it width: 100% to see what I’m talking about)
p.s. Theme in the link is a little outdated version, so I’m asking only about vertical featured image, other issues that you may see have been fixed.
I have only one approved wp theme and this is what did for featured images. I have set display to block, margin to 0 auto, max-width to 100% and height to auto. I have also added a texture in background 
Don’t force width 100%…instead set your widest thumb image size to cover 100% of your theme viewport. To see how you should handle the vertical image just activate one of WP default themes. Image itself has max-width: 100%; and height: auto;