"Set your default programs" is subordinate to "Default programs". No sign of Picture Manager.
"Change Autoplay settings" has a setting for "Import pictures and videos using Windows" as opposed to "... using Dropbox". Dropbox was the incumbent setting. Looks like a good place to start, thank you.
FWIW, all the Help items on the subject did not use the name "Picture Manager". I wonder if there is a distinction between Picture Manager and Windows in this context?
P.S. It works just fine! Thanks for the help!