The career of thirtysomething shutterbug Melissa Steadman (Melanie Mayron) has been shooting skyward lately: First she snapped Carly Simon's album cover, then she landed a major magazine feature, then an art dealer mounted a retrospective of her work. All of which is great news for 37-year-old Deborah Roundtree, the California photographer who takes the pictures that fill Melissa's portfolio. Roundtree got her break through Chiat/Day/Mojo, the ad agency that vets the show's plot lines for authenticity. ''It's been wonderful,'' says Roundtree, who recently shot several of Melissa's ex-boyfriends for a ''gallery of failed relationships.'' ''I use a lot of black-and-white,'' she comments, ''and try to bring a little mystery to it.'' That isn't hard, since Roundtree doesn't follow her on-screen counterpart's life too closely. ''To tell you the truth,'' she says, ''I don't watch. But sometimes I have a friend tape it.''