Step 1: Install the Devel module (http://drupal.org/project/devel) and put "dpm($node)" at the top of your node-product.tpl.php. This will show u every property in the node object, and "how to get there", including the necessary parent property / array keys.
Step 2: If above doesn't solve the "which is the right field" problem, you could try and install http://drupal.org/project/filefield_sources which extends the imagefield with a link to the filesystem, next to the upload field. This way you could select existing images from G2 as your product image. It automagically integrates with the existing imagecache_field from Ubercart.