How to specify image size for post header image preview in Academic theme

I know how to use … css stuff within a markdown post in the Academic theme to specify the size of an image used for the header of a post:

img { height: auto; max-width: 70px; margin-left: auto; margin-right: auto; display: block; }

But when looking at the site front page preview of recent posts, the image is huge, taking the entire window width. How can I limit the size of the preview thumbnail image? My site is if anyone wants to take a look.