Clue 3 hints:
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The romantic city is in Italy born of water.
Shenron is a dragon. A dragon had a battle with a Saint. There is a famous painting you must find. Where is it housed?
When you are on the google map of Venice use the 'nearby' function and enter the art gallery name. Go on street view and enter inside the gallery. OR go to art gallery website and take the short (about 40 seconds) virtual tour. What you seek is upstairs - a flag.