Currently, it looks like the recommended size for a thumbnail is 229 x 173px - with an aspect ratio of 1.3:1 - and the recommended size for a News Article image is 310 x 310px - with an aspect ratio of 1:1. Because of this difference, and because the News Article image is used as the thumbnail for the News Article, the thumbnail will be stretched to fit the aspect ratio. I'd recommend either updating the News Article recommended size to be 229 x 173px OR give us the ability to upload a News Article image and a thumbnail as separate images, similarly to how Courses give you an option to upload a UI3 thumbnail, thumbnail, and poster.