get_post_thumbnail – jquery slideshow

I have a theme that I am working with (https://www.blackswanscreenprinting.com) that is pulling a thumbnail image and a large image into a jquery featured content slideshow. It is pulling both images from custom fields. I am trying to get the thumbnail to pull from the featured image field. I have been working on this one for about hours and need help. I did not write the original code and it is confusing me.

Can anyone figure this one out for me?

[code moderated as per forum rules - please use the pastebin.
should you post a small amount of code directly, please mark it using the 'code' button - highlight the code and press the 'code' button once; or press the 'code' button, then wrtie/paste the code, then press the 'code' button again.]

Much obliged.