If you have a digital camera with a TV-out capability (like s-video or composite video) you can plug the camera directly into the TV's inputs and view your pictures that way simply by turning on your camera and reviewing photos. Most of the time that's the easiest way to do it, and you can control the picture viewing directly from the camera.
Or, if you have a DVD player hooked up to the TV, you can also burn a CD-R data CD containing all of the pictures you want them to see, throw it in your DVD player, and hitting 'next chapter' on the remote would flip through the pictures. But you'd need to make sure your DVD player supports CD-R data CDs.
Also, depending on your cable provider, if you have a STB (set top box) from Motorola or DirectTV, you can insert digital camera memory cards into the box, and the STB will output directly to your TV just like you were watching a TV show.
Otherwise, like you said, you might need a 3rd party device that allows the insertion of an SD memory card with pictures on it or something of that nature.
Hope this helps!