Melt semi-sweet chocolate according to package directions. Pour melted chocolate over the bark, then spread to evenly cover the bark. Melt the white chocolate, then use to make the ghosts. Just spoon dollops (1 teaspoon for each small ghost, and 1 tablespoon for each large ghost) of white chocolate, about 1 inch apart, onto the semi-sweet chocolate. Use a toothpick or skewer to drag the white chocolate into ghost shapes.