Classifieds: Featured image not setting!

I've recently had issues getting Classifieds working. I decided to entirely change themes to get it going, and it seemed to do the trick. However, featured images are not setting.

Please see the following post for screenshots. As you'll see the Add Classified form displays correctly, and the second screenshot shows the Media Library popup and my attempt to set a Featured Image.

However - when I choose an image (i've tried all shapes and sizes), the Media Library simply returns me to the form and the image i've chosen does not appear where it is supposed to - for some reason it's not allowing me to SET the featured image, or not recognising my attempt to do so.

Please advise - thank you.