[Theme: Hatch] Featured Image In Single Post

Hi,
I’m using the Hatch theme for my WordPress site relating to graphic design. Each post I include contains one image and a brief description/link.

When a featured image is clicked from the front page, the single post then displays both the featured image AND the full sized image. I would like to prevent this from happening, so that the single post displays ONLY the full sized image.

Please see: http://www.perkinsdesigns.com/imagination/

I can see that many people using the Hatch theme do not have this double-display issue, but I cannot see where or what changes need to be made?

Any help is appreciated, thanks.