Show Featured Image on Post Page

Hello, I've been searching and read some past posts about this topic, but I feel like there has to be a way to make my featured images show on post pages. I love this theme and just purchased and configured it to my site last night, but I'd really prefer to not have to go through 1,500 posts just to add the same featured image to the top of the body for it to show on the post page. Is there some css I could add to fix this, or maybe a plugin that could suffice? Please let me know. My site is www.nerdunderground.net