How to make featured image centered

I have tried everything and read too much on how to do this and it seems like I can’t ever get it right. I have tried some things that have either messed up my blog or I got the blank screen of death. I don’t want to do something in my code that will simply just move the margins or something- because I might have to change that again later? Or cause other problems?

Here is a link to one of my posts without a centered feature image. You can not really tell that much, but I can. And I want to change this for future posts, instead of just simply not having a featured image