Hi Jay, i have downloaded the theme and tested it, and i didn’t have any problems with the post images displaying in the slider. Also, when i entered to your site, i can see only one post with it’s image in the slider, in the correct size.
Please, when you add an image to a post, don’t forget to click on “Use as featured image” link after uploading the image, as our instructions say:
“…After WordPress has finished uploading and crunching it, a panel will slide down with options regarding such image. Scroll down to the bottom of it, to find an Use as featured image link, which you need to click. Save all changes afterwards,…”