When you make posts in most forums, it adds to your postcount. When your postcount reaches a certain number, you rank up and the picture changes.
Posts in Miscellaneous Polls, SPPf Help & Newbie Lounge, Serebii.net Discussion, Chatroom Discussion, Other Pokémon Games, Webmasters and Websites, Clubs, Games, Role-Playing Games and Tech Help do not increase your postcount.