Hello John:
I forgot to mention that. I did disable the (WikiSEO) extension this morning. The problem with the images recurred after I disabled it. Also, I haven't had code in the wiki that would call the WikiSEO extension since August 28. So, I am doubtful that this is the underlying cause.
I have considered taking down the citation generator... but I am doubtful the problem lies there as it is a stand alone application. Beyond that, I have awstats installed -- but that install precedes the problem by several weeks and also is unrelated to MediaWiki.
Above stated, I have to do more reading to find more ideas about what could be wrong. PHP memory settings is one idea. Memory something is my hunch at the moment.
I will reboot the server when I have a chance -- see if that gets back the images. I suspect this will temporarily make it seem the problem is gone.
Thanks, Michael