The benefits of a Premium account over a regular account are:
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.
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.
The prices for a Premium account are:
You can have the Premium account for 1 month for free! Just activate the trial period. You can do it after you log in - click Settings, select the Payments tab and click the "Activate Trial" button.
$5 for 1 month
$18 for 6 months (equivalent to $3 per month)
$24 for 12 months (equivalent to $2 per month)