The premium account has everything the free version has plus:
- There are no ads in the gallery pages. In the regular accounts there is one Google AdSense ad in every gallery page.
- One extra gallery type is enabled. It allows you to embed large photos directly in the item page. There is no need to click on a thumbnail to see a large photo.
Note: a "gallery page" is the page that you see when you click on a thumbnail. It contains all the thumbnails and a large version of the picture you clicked.
Maybe you didn't need the premium features so you didn't notice the differences. In that case you just don't need to pay:)