Featured images on posts page

Hi,
First of all, lovely theme you guys made here, great job.But I’m having a slight problem.
On my posts page, I’d like my posts to have featured images (just like the theme’s demo). I’ve tried using the featured image option, which does allow me to do it but the problem comes in where you actually go to the post and the featured image’s size is up scaled and ends up ruining the outlook of the post.
Is there a way to have the featured image appear only on the posts page but when you click on its post link , it doesn’t show on the posts page?
Will really appreciate the help please!