How do you keep the original picture size of featured images?

Hi, I am using the Social theme and is trying to use the Theme options --> Gallery settings ---> Article feature.
And everytime we make a post with a picture, the picture always gets stretched out on the homepage on the width to fit the borders.
So the question is: How do I prevent it from making the image larger?

It found this field but I am not sure if it is the right one or how to change it: "Change your desired custom field for featured post images
Default: thumbs"

Cheers
/Robert