How to add sizes for "Wallpapers" section?

I own Hoops-Nation.com and would love to add a “Wallpapers” page, where we consistently post new wallpapers.

I ran across this magnificent site: http://posterizes.com/portfolio/hakeem-olajuwon-wallpaper/

Below their wallpapers, you can click on any size and it loads a new page with the wallpaper. Can someone please explain to me how that is done?