Last month, Insight contributor Lev Kushner suggested that San Francisco needed a flag that better conveyed the 21st century spirit of the city. “The way a city represents itself graphically is important, both because of the values it communicates and because of its ability to rally disparate people under one banner,” he wrote. Here is a sampling of readers’ ideas for such a flag. To comment, submit a letter to the editor at SFChronicle.com/letters.