I believe that the issue here is that Walmart does not send the image url with the order. However, if you add the url of your images into the Products section of Shipstation, the images will be displayed on your Wal-mart orders. We have done this and it has fixed this issue for us. If we have a new product that sells on walmart that we haven't imported, we just add the image url at that time. Hope this helps.
Jeff
Other developers have figured out a way to add images to Walmart orders without us having to manually import them... has ShipStation figured this out yet? Please make it a priority.