Banners are changed by the staff, it is not automatic. All users see the same banners at the same time.
Users submit banners via E-mail to the site staff, and if it happens to be selected, it gets used. Everybody is capable of sending in a banner, so it's not an exclusive thing.
To see who made the current banner, look towards the bottom left of the banner itself, for example, you can see that the current one was made by Rascal61.
There is no way to change the banners back, though, sorry about that.
I hope this helped.