From what I am seeing, the theme let’s the featured photo take up all available width, so if you have a wide browser and screen, then it stretches the photo. Again, I’ve seen other people bring up this issue.
When I remove my custom CSS and look at my blog in a browser that is maximized, the featured image is stretched beyond 700 px, which causes distortion. I’m just not sure why you resize an image and then let it display so wide — why not just not resize the width, or not have it take up more than 100% of its width?