The peace symbol was a symbol in 20th century Earth culture.
A group of protesters seen in the resetting time stream carried a large peace symbol. (ENT: "Storm Front, Part II")
In 1996, Rain Robinson had a peace symbol sticker on the driver side window of her van. (VOY: "Future's End", "Future's End, Part II")
The picture in the time stream was taken during a April 1, 1971 protest in Washington, DC. 
The peace sign seen on Robinson's car is a design combining it with the Ecology Flag