Who does the voting and is it only done on that first page where they change the photo's?

Anyone can rate a tank, either through the home page, or the ratings pages (once you rate a tank through the home page, you're taken to the ratings page).

You have to go through the pages you mentioned - you can't find and vote on a specific tank, as this tends to get abused and skews the votes (i.e., people vote multiple times on the same tank, get groups to vote up/down certain tanks, etc.).