Bring sugar and water to a boil in a saucepan, stirring until sugar has dissolved. Stir in lavender. Remove from heat, and let stand for 10 minutes. Strain, discarding lavender. Let cool. Stir in lemon juice. (Lemonade can be refrigerated in an airtight container for up to 1 week.) Serve over ice, and garnish with lemon slices and fresh lavender sprigs.