Finely chop half the olives, and thinly slice remaining; set aside. Remove yolks from eggs, and discard. Place whites in a medium bowl; using a fork, mash egg whites until 1/4-inch pieces are formed. Add mayonnaise, tuna, olives, capers, and chopped tarragon; stir to combine. Season with salt and pepper. Top crostini with mixture; garnish with tarragon.