It appears that your site is one hosted with WP.COM, please visit this page and click on Support or Forums.
http://en.support.wordpress.com/com-vs-org/
This level of modification involves changing image sizes in theme function too. So you need to create a child theme for it, but WP.COM managed host wouldn’t allow anything more than Custom CSS.
Anyway, this request is possible with CSS-only but the downside is that images in front page will get enlarged with CSS from it’s original size, so it will lose some clarity.