Where's the Picture?
If you can't enlarge a single picture ...
If “click to enlarge” worked the first time you tried it ...
Here's how it works:
If you close one popup window with an enlarged picture in it, and later click another picture to enlarge that one, a new window pops up with the new enlarged picture, and all is well.
But if you don't close a popup window, it disappears from sight the moment you click something else (for example, the scroll bar or the next picture you want to enlarge).
Although you can't see it, however, the window hasn't closed. That last click was on a different window (which is how the computer regards your main browser screen), so that window has been moved to the top. If you minimize the browser screen, you'll see the enlarged-picture window right where it was before the bigger screen covered it.
If you've clicked another picture in the meantime, the enlarged version of that second picture (or the last picture you clicked, if there were several) is what you'll see in the uncovered window.
For best results, you should always close an enlarged picture window before you try to open another. That's much more convenient than constantly minimizing the browser screen.
Besides, window shapes don't change automatically. So, if you click a wide picture while a narrow window is still open, the browser stuffs the new picture into the open window regardless of the bad fit, and you'll be able to see only part of it.
To avoid such problems, get in the habit of closing each popup window as soon as you're finished with it.
Why Is the Website Set Up That Way?
(However, it did all the basic stuff pretty well, and it was cheap.)