In order to get an accurate and full understanding of the problem, for some, I took this to the site admin team through the moderators link. This is the response of the vBulletin tech support team,
"So, the site has a resizer plugin on it that max's the width of all pictures to 600 pixels. Giving you the option to click the image to see full size. The issue is if you use adblock or other noscript add-ons it will block the plugin causing you to see basically all or nothing depending on the source. Does that make sense?
This was my response to Kyle,
"Yes, that is a perfectly clear explanation. Adblocking add-ons are the source of the display width problem which is reported by some. This is an easy fix for those who use adblocker, like me, since there are controls to disable it entirely or for specific sites.
Thank you very much for your help. In the future I will refrain from enforcing or attempting to enforce picture/graphic width.
Thank you very much for your timely response."
So the quick bottom line is that the overwide pages that I sometimes see, and that some others see, is most likely caused by my adblocking s/w. As stated above, the problem is easily fixed on an individual basis. Thanks to S. Keith and to samwichse who provided testing and clues