The right location is inside Lake Ferona Cave, at coordinates 1930, 1320, 0163.
They want to see a picture of a full moon that's visible during the day, which should be somewhere near Lake Ferona. To start this side quest, go to the center of Goron City and speak to the two Gorons in front of the Inn.
To start Moon-Gazing Gorons, make sure you've completed the Yunobo of Goron City quest.
