Hello David,
Thanks for that info and choosing the Apple Support Communities. I know how important syncing your pictures to your iPhone is to you!
If I understand correctly, you are unable to select the Photos app in iTunes on your Mac as part of the iPhone syncing process. Let’s start with these easy steps to help resolve this issue:
1. First, restart your Mac, as well as your iPhone.
Restart your iPhone, iPad, or iPod touch
How to turn on or turn off your Mac
2. Next, make sure you have all available OS X, iTunes and Photos updates installed
OS X El Capitan: Keep your Mac up to date
OS X El Capitan: Install, update, and uninstall apps
3. Then try your sync via USB again. Make Photos is open while iTunes is open, before you connect your iPhone.
Sync your iPhone, iPad, or iPod touch with iTunes on your computer using USB
Cheers!