The answer looks to be that the CMS framework around the site is causing the issue. I am using a HTML plugin on a wiki page to display this information. Everything has worked fine using this strategy and it didn't occur to me that the outer framework could cause this one singular problem of image resizing on mobile until I saw these suggestions confirming that previous attempts should have worked fine. I copied my HTML to a stand-alone page and it works fine. So now I have a different kind of problem to solve.

At the risk of TMI I would like to thank you for hanging in there with me to figure this out. I used to run highly technical teams in a very hand on way as a go-to sort of guy and am now in my 4th year of recovery from a brain injury. I am at a point now I am trying to make myself useful again. This may have seemed like a waste of time for you, but for me it has been more helpful, and meaningful, than one might guess. So thank you again for your help.