Featured Image Blurry

Currently using Poseidon Theme for my blog. All of the featured images on my posts are very slightly blurry. Have taken them with a good camera and uploaded in at least 960×640 dimensions, so don’t understand why the featured image isn’t showing correctly. Photos in the body of my posts are fine, even in cases where the same image has been used (see attached link).

Any help would be greatly appreciated!

The page I need help with: [log in to see the link]