Can't Change Post Widtch

Home url for my site is http://www.wadeedwardsart.com/
If you click on one of the images on the home page it will bring you to the related post for that painting, for example http://www.wadeedwardsart.com/corvoidea-eternal/

I want the image on the post to be 960px wide so it fills up more of the screen. I’ve tried adding .post {width …} but that also changes the size of the thumbnails on the home page so I completely lose the effect I currently have on the home page. So, I want the home page to stay exactly the same but change the width of the paintings on posts. The larger image in the posts is currently a Thumbnail in the custom fields. I assume I would have to add the larger painting images into the posts themselves once I have widened the posts, instead of leaving them as thumbnails? Hope that’s clear enough. I would really rather just add some css to the styles.css file if that’s possible to achieve this as I’m not that strong with code. Thanks.