Problem sizing the images in post content

Hi,

I am using the massively theme. When I am trying to post a vertical image, no matter what editor I choose, I can’t resize it. Or rather I can resize it in the editor but when previewing the page, the image is displayed on full size.
The resizing for vertical images only works when I am doing a left or right aligned image but not with a centered image. I’d like to center an image and resize it so it doesn’t take the whole space, is that possible ?

Thank you, I hope I am being clear.