Each page has a group of pictures with some text. If you click on a picture, a large window will open with a larger copy of the picture in it. To return to the previous page, minimize this window. (Minimize means click the - sign in the upper right with the mouse)
With some browsers, once this window has been minimized, you must "restore" it yourself. If you select a large picture and nothing seems to happen, double click on the browser icon on the bottom of your screen. The large window should appear with the picture you selected.
Some of the large pictures will open in a "normal" window with a "back" button. In this case use the back button to get back to the previous page.