I can't say specifically what's wrong with your listing, but I would preview your listing in Auctiva, make any necessary changes so it comes out right, and then re-paste your template HTML back into your listing in ebay.
Try setting up your template using only the "design editor" mode, rather than designing directly with HTML, as often doing your own HTML seems to be the source of problems. If you are selecting your images on the one page lister, they should automatically display in your template at 400x300, no problems. If you are manually inserting them with HTML, well........ (If you manually insert a URL of a pic, it will display the pic at whatever size you have saved, for example when clicking a supersize link. You would have to downsize your saved image to make it display smaller. If you want to keep the "thumbnail to supersize" feature, let it display automatically as mentioned above)
Once you have it right in preview, try pasting your HTML into www.practiceboard.com until you get it right, then paste in ebay.
This advice is from a non-HTML knowledgable perspective. Obviously an HTML person can give you more technical help.