featured image not showing at 100% width

http://www.thelovelustlist.co.uk/blog/win-your-wedding-photography-with-a-thing-like-that/

the blog is set to be 940px wide – as you can see the featured image is not showing at full width… the same image is placed in the post and you can see it extends the full width of the page… can you let me know how I Can correct this?

Thanks,

Adam.