Arras Theme Thumbnails not working

I am having a hell of a time getting thumbnails to work on my blog with this theme…I just get the missing link ‘?’ icon on the front page and nothing comes up in the big slide show.
I followed the instructions…upload image, copy file URL, create new custom field, name it ‘thumb’, add the url (https://www.edenweb.net/wp-content/uploads/2010/02/thumb11.jpg) but nothing.

Can anyone help? The image shows up in the story, but ideally I would want it only in the thumbnail and the home page slide show.

I’m using a Mac, but I am assuming this shouldn’t affect anything?