Olympic gymnast and Needham native Aly Raisman will help fight against eating disorders through a partnership with Walden Behavioral Care, according to officials from the private psychiatric hospital in Waltham. Raisman, who will serve as a spokeswoman for the facility, said in an announcement issued by Walden Behavorial Care that she has known individuals battling eating disorders, and has seen firsthand the effect that they can have on a person. Raisman was chosen as a spokeswoman because of her positive perspective on self-esteem and body image, her emphasis on a healthy and active lifestyle, and her interest and passion in encouraging people to be confident and make smart, healthy choices, according to Walden officials. As part of her partnership, Raisman will make appearances at Walden events, both for patients and the public. She will also star in public service announcements, which will help to educate the public about the dangers of eating disorders, officials said.