Toolbar options
In the upper right hand of the screen I've seen two opposing arrows on other Macs that make the image full screen. I cant seem to find that on my MacBook Pro
MacBook Pro, Mac OS X (10.6.8)
The item you are talking about is a feature of OSX 10.7.x (Lion) and is not part of Snow Leopard. Some apps may support Full screen mode in SL like iPhoto or Firefox, but they will have their own method for switching. Lion added the arrows icon in the corner method to standardize the interface.