Hello Andyec -
This is an API error that is returned by eBay most typically when the listing you are attempting to post does not contain a Header/Gallery image. There are a number of things that can occur during the listing process, depending on the order that you do things while creating a listing that could cause the listing to not have an appropriate image in the Header and Gallery locations in the Image Selection area of the lister. Please review the Saved Listing which has received this error and apply the appropriate image to the Header and Gallery image locations on the right side of the image selection area. Once these are present in the listing, there should be no related difficulty with it posting.
If you have a listing that has receive this error where the images are there, please file a support case from under the Help tab of the site and let us know which listing or listings are encountering trouble and we'll be happy to look into it for you.