A quick explanation from me would be:
Just click on any picture in a thread and then either click on the picture and another will pop up. You can keep going until you have seen them all, just click and view all the pictures on that post thread. Or keep going forever.
There is also an arrow at the bottom of the picture which allows you to see how many pictures there are and to move forward and back through the slideshow.
Sorry Sharon I did not think that thread explained it quickly.