Thanks for your suggestions and comments.
Looking from cPanel, I’m seeing fully populated images folders in the forum themes. The problem persists after switching to the stacked theme and then back to css-only theme.
When I view the Chrome console, I can click on one of the image links in the list of 404 errors. This brings me to a Page not Found 404 error within my site http://mysitename.com/wp-content/sp-resources/forum-plugins/subscriptions/resources/images/sp-SubscriptionsReviewButton.png
However, when I shorten the above to the containing directory rather than the file, http://mysitename.com/wp-content/sp-resources/forum-plugins/subscriptions/resources/images/, I am presented a list of files in the directory. Oddly, though, these are NOT the filenames that I expect to see here, and that I do see in cPanel. Here are the files (and directories) shown at http://mysitename.com/wp-content/sp-resources/forum-plugins/subscriptions/resources/images/:
Parent Directory
mobile/
sp_SubscriptionsForumStatus.png
sp_SubscriptionsPostEditor.png
sp_SubscriptionsReviewButton.png
sp_SubscriptionsSubscribeButton.png
sp_SubscriptionsSubscribeForum.png
sp_SubscriptionsSubscribeForumButton.png
sp_SubscriptionsUnread.png
sp_SubscriptionsUnsubscribe.png
sp_SubscriptionsUnsubscribeButton.png
sp_SubscriptionsUnsubscribeForum.png
sp_SubscriptionsUnsubscribeForumButton.png
I’ll keep exploring and let you know if I find anything else of interest.
Brad