Changing an image on the homepage

Hello,

I am need of some assistance to replace one of the 4 "icons/buttons" on this home page http://cedarparkchiro.com/.

Specifically the New Patient Forms image however I have not been successful in doing so. It appears the theme was originally designed to pull the featured image from posts within a specific category. However, when I go to edit that specific post, it actually shows a different image than is currently displayed so I'm not sure how to edit this.

Thanks for the help!