maybe I can help out while shabu is writing that guide. if you havn't already figured it out, when you write a post you upload your image (which should be a large image to be seen on the single post page). click the radial button that says thumbnail, and select insert image into post. then it will give you the full url of your large original image, and it will also auto create a thumbnail image for you and give you that url as well . copy both, and then go down to the custom fields section and create a new custom field and call it thumbs. then paste the thumbnail url into the new field you created. then make another field and call it image, and paste the url of the full size image. save your post, publish it, and that should work. hope this helped
