MisterArt.com will announce tomorrow that Turner Broadcasting chief operating officer Steven J. Heyer has joined the online art supply store's board of directors. Heyer, who also serves on the boards of Internet incubator eHatchery and RealEstate.com, is the third addition to MisterArt.com's board this year. Mindspring president Mike McQuary and money manager Fayez Sarofim joined the company's board in January.