Stir together pecans and margarine in a small bowl. Spoon equal amounts of mixture into paper-lined muffin tins, pressing evenly. Soften gelatin in water in a small saucepan; stir over low heat until dissolved. In a large bowl, beat cream cheese, milk, and sugar at medium speed of an electric mixer until well blended. Stir in gelatin. Pour equal amounts into muffin tins; freeze until firm. Place peach slices in food processor or blender; process until smooth. Just before serving, spoon peach puree onto individual plates. Remove cheesecakes from freezer 10 minutes before serving; peel off paper. Invert cheesecakes onto top of peach puree on each plate. Garnish with additional peach slices and fresh mint leaves, if desired.