Blog Post Featured Image Distorted on Mobile Devices

Hi there, The featured image at the top of blog posts appears distorted on mobile devices when in portrait orientation. Tablet and desktop display fine. Can you please advise custom CSS to fix this.

Alternatively can you advise what I need to add to the below CSS if I want to hide the featured image on mobile devices as well.

I would prefer first option though. Thanks in advance.

.single-post .img-polaroid {
display: none;
}