Arras problems with the thumbnail option

Hi

I am trying to use wordpress as a review site and found this theme which I am trying to get my head around. One of the features of this theme is to show a picture of the reviewed item/article as the main headline.

Reading on how to do this, I have tried two ways, the 1st as mentioned in the guide which is to insert an image get its file url , create a custom field called thumb and enter the file url into the value. This did not work even though the image was 700×300 as recommended.

I have also tried this but by adding the image into the media library 1st and then getting the file url from there but no joy . The version of the arras theme is 1.3.5

Cheers

TimCS