You're showing two separate things. The title says the volts go to 17 when the engine is running, as opposed to a photo showing <8 vdc.
The voltage should never be less than the charge on the selected battery, so it sounds like you may have a dead battery that the alternator is trying to charge. If the alternator/regulator are not already failing, trying to charge a failed battery can do damage to the alt/reg before long so I'd suggest:
First use a hydrometer (preferably) or your multimeter on each battery with everything off. The benefit of an inexpensive hydrometer is that it can isolate whether there is a dead or weak cell. The multimeter indication is at battery level.
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If you've not already found the voltmeter is bad, then they will probably need a charge even if not needing replacement because of your voltmeter photo. If a battery cell or charge was found to be low, take it (them) to a car parts store for a load check to verify the capability (health) of each battery.
Now that the heart of the system is known good, re-install and start the motor then see what the multimeter shows for the voltage across each battery while charging. Anything at this point is in the alt/reg, or wiring.