By Emily Mitchell/Reported by Wendy Cole

"The roses were in my imagination," says William Britt. The artist, 54, who spent his first 34 years in New York City's Willowbrook institution for the mentally retarded, was commissioned to paint Rose Kennedy's Hyannis Port, Mass., home for her 100th birthday on July 22. Britt is in a program founded by Jean Kennedy Smith that supports art by the disabled. The matriarch is being presented with the painting this week, and its proud creator rehearsed what he'd say: "Thank you very much and happy birthday."