A Gobo Light is made with metal or glass discs that have a design cut into them. When put in a spot light/profile between the lenses and the lamp, the result is a projected image of the said design.

You can create the same effect using IRender nXt by placing a Spot Light or Point Light in a cylinder, and placing geometry or an alpha-transparent image on the face at the end of the cylinder.

Make the light itself very small in order to create sharp shadows, (or larger to create software shadows)

For the image below, the spot light is 1" wide (The 3' visual beam appears larger, but is just for positioning. It is not part of the actual light)

The spotlight is placed in a cylinder (semi-transparent in the SketchUp X-ray view so you can see it better) in a cylinder with an alpha-transparent image on the end face of the cylinder (The SketchUp Fence material)