I will be taking a 3-hour tour of Venice with a professional photographer. What would be an appropriate amount to tip?
Zero. Italy is not a tipping culture. Shake his hand at the end and say "Thank you." If you enjoyed the tour, tell him you're going to be leaving a positive review of him on TripAdvisor and/or that you will be recommending him to friends. Then walk away.
Thank you, Michael! We will follow your suggestions.