Featured photo issue

The featured photo for each of my posts is stretched and I would prefer just to not have it show up in each post. It is also stretched when you click on the monthly archive as opposed to the other listed posts, which appear fine.

If I turn off featured image, I get a broken image icon instead of the featured image in the posts lists. Obviously, this isn’t good either.

Furthermore, the background image is pushed halfway down the screen when you click to the monthly archive list. Reloading the page fixes it, but my customers won’t know that.