Unlike in Tiger where you can do image sorting with simple click anywhere on the sidebar - as discussed in my earlier post Meet the Preview Ultimate Sort Ability, in order to perform images sorting in Leopard Preview you need to click on the gear button (to the left of the slider), expand the menu and choose Sort By.

I like to see the animation running by Leopard when arranging all images in their sorted order (unfortunately, you cannot do slow-motion for this sorting animation).