Welcome to the community, @simonwillans! Could you kindly share with the community a screenshot of what you are able to see and what you are not able to see? Maybe this should help the community to understand your issue pictorially.
In edit mode the pictures are visible and in open mode the error states that kobo could not find the picture. Please ignore the actual picture numbers, I quickly grabbed screen shots from different site but the error outlined exists in all cases