Energy,
I don't know why your picturs wouldn't be showing in the thumbnails but showing in the auction if I'm understanding you correctly.
I'm assuming your trying to edit a saved listing that was listed through Auctiva. Right?
Go to Saved Listings, Click on the listing you wish to edit. Change title and other text you wish to revise.
Go to Image Selector(where your thumbnails are) and if there are pictures there, underneath each one it should say edit or remove. Click remove on each thumbnail and replace. (Make sure your pictures have been uploaded to Auctiva).
If the thumbnail picture is no longer there, trying clicking on the little box anyway and see if it will take you to where you can upload or if you've already uploaded them to Auctiva you should be able to see them & choose.
Perhaps you've already removed them or something ? I don't understand why they wouldn't be there.
After you have revised your listing don't forget to click SAVE (or post) at the bottom. I usually click save first and then go back & look at the listing before posting to eBay to make sure I have everything revised the way I want it before posting it.
Hope that makes sense and if I'm misunderstanding what's happening in your case let me know. Also, you could have Auctiva take a look at those thumbnails & see if it's a glitch or what.
Good luck.