I have an iMac G3 and use Internet Explorer.  Occasionally, when I am
clicking on links to (or viewing pages of)  photos, some images fail
to display.  Instead, a button-like red circle with a white "X" in the
center appears.  In some cases, clicking on the "button" will open a
larger photo; more often, nothing happens.  How can I correct this
problem?  Are there plugins that would enable me to view these photos?

Generally, that symbol means the image is not available where it is
supposed to be. As for the times when you click and get a different
sized image, it sounds like the full image may be where it is supposed
to be, but the thumbnail sized image is not.  Just a guess.

An easy way to see if, as hansg-ga suggested, the image file is not
where it is supposed to be is to view the page's source and find the
reference to the image.  If it is there, you could just be using an
older version of IE which couldn't display some types of images.  It
that is the case, or even if it isn't, the best thing to do would be
to use one of the many better browsers now available for the Mac. 
Such as:

Mozilla
Camino
Safari
Omniweb
Even the latest version of Netscape, based on a version of Mozilla
about 6 months old far exceeds IE at this point.

Thanks, hansg and leoj!  Since this seems to occur mainly when I am
accessing archived auction listings, that would explain the problem:
the image is simply unavailable, rather than unviewable due to some
browser issue.  So, case closed, I suppose.  ( And yes, I must admit
that it's probably time to upgrade.)