If you don't have any idea about Terragen, look here first. More information and downloads on Terragen are available at Planetside-Forums.

The contests take place monthly. In the navigation menu Contest Index you can see an overview of all running contests. Moreover all current contest images can be viewed. If you would like to participate or vote for images, you have to create an user account. The registration only takes one minute and is completely gratis and risk-free... the results of the contests are viewable for everybody.