An image in your listing will only get larger if you have uploaded it at a size greater than 400x300 pixels. (The standard eBay size).
Try loading them at twice the default size and you should see the difference clearly.
When your listing is posted via Auctiva with supersize option selected it wil have text under it referring to supersize.
The 'save/download' window you saw is a Microsoft Windows menu not an Auctiva 'Window'.
You do not need to know about HTML for supersizing.
A white page image (where your item picture should be) with a red cross in it means that your image could not be found. Could be that the host (eBay or Auctiva or someone else) has not been set up correctly in yor Auctiva settings. You have the choice of image host.
Sometimes the X will appear if the image host is bogged down with traffic and did not respond in time.
In your case where the X got bigger means that supersize appeared to be working so it just remains to connect the place your image is on to Auctiva.
Sounds like you are nearly there so chin up