After giving up and working on unrelated things, the fix came to me... Using the schedule tab under the camera, I created an event. I initially picked snapshot, but should have picked "trigger". Then under the alert tab, create a new "on alert" function that sends email - but only with external trigger (uncheck everything else, including motion). Then when the scheduled time arrives, the scheduled trigger fires that "on alert" function to send a single picture via email. Works perfect.
I appreciate the python routine, and ultimately, if this hadn't worked, I may have been forced to attempt that. As it is, I'm glad to keep it within BI.