Most clans used to have banners of their servers on their website provided by GameTracker. After GameSpy had cut its support for SWAT4, GameTracker was no longer able to scan the servers. As an alternative you can use service offered by WWW.GAME-STATE.EU. Though being not as rich in features, it can provide the basic info about your server for your website including some colour customization and a number of banner designs to choose from. One example is below, to see all possible designs, click here.
After registration, you can add your own servers. However, mysteriously some cannot be added and some are not scanned properly. Also forget player statistics, server rankings, map images in banners, etc.
EDIT: GameTracker support available again - read more here.
EDIT: GameTracker support available again - read more here.