An isosceles triangle could have rotational symmetry if it were also an equilateral triangle. An isosceles triangle is a triangle with at least two equal sides. An equilateral triangle is a triangle with exactly three equal sides. By definition, all equilateral triangles are also isosceles triangles.
A shape or image with rotational symmetry is one that can be rotated by some degree amount and still appear the same. In the case of an equilateral triangle, the shape remains the same each time the triangle is rotated by 120 degrees. As a subset of isosceles triangles, equilateral triangles have rotational symmetry.