Update. I was able to inform Yahoo support about the issue. Two (3 actually) things came of it:
1) The support rep was able to reproduce the issue as I described it, and they passed it to engineering.
2) I became aware of a workaround (**)
3) I'm in complete shock (but happy) that, from a giant faceless monolith like Yahoo, a person actually engaged a user's issue, rather than spewing canned advice like, restart the app, reboot the phone etc.
** Workaround: Select pictures to upload within the Flickr app, rather than within Photos.
Specifically: Activate the Flickr App. Tap the camera icon on the bottom middle of the screen. (This activates the iphone camera.) Tap the icon in the bottom left corner of the screen that looks like a photo thumbnail. (This accesses photo albums on the iphone.) Select pictures. Tap "Done" when finished selecting then tap Post or Upload to start the upload.
It's not a perfect workaround, because there's no select-all ability in the Flickr selection function, so if you've taken a large number of pictures in a day and you want to upload them all, it's a bit of a PITA. But at least all selected pictures upload this way, whether it's 10 or 100.