I also wasn't getting a thumbnail displayed, and solved it by

1. disabling Lightbox2
2. enabling Imagecache 5.x-1.5 (I assumed Lightbox2 meant I didn't need it)
3. Going to Store Admin and acknowledging the Status message regarding Images.

Now I just need to work out how to configure the items on the /drupal/products page to:

• show product details and Add to cart button
• Make the thumbnail clickable to show a larger version