Post thumbnail sizing issue

When I add a featured image/post thumbnail (NOT a header image) to a post, no matter what size the upload is it automatically stretches to become a massive, screen-sized image at the top of every post. I’ve tried to add the set_post_thumbnail_size code to my functions sheet, but haven’t had any change. I’m very new at all of this, so I might be adding the code to the wrong place. I just don’t know and can’t find what setting to change or what code I need to add and where to add it after reading all the forums and help topic pages.

Here’s a sample – http://focuspressblog.com/2014/06/19/test/

[Duplicate: https://ww.wp.xz.cn/support/topic/featured-image-oversized-2?replies=1%5D