Button btn = Images[k].GetComponent<Button>();
if (btn != null)
{
    if (btn.transition == Selectable.Transition.SpriteSwap)
    {
        int counter = 0;
        SpriteState state = btn.spriteState;//通过这个属性获取，是一个结构体
        if (state.highlightedSprite == a)
        {
            counter++;
            state.highlightedSprite = b;
        }

        if (state.pressedSprite == a)
        {
            counter++;
            state.pressedSprite = b;
        }

        if (state.selectedSprite == a)
        {
            counter++;
            state.selectedSprite = b;
        }

        if (state.disabledSprite == a)
        {
            counter++;
            state.disabledSprite = b;
        }

        if (counter > 0)
        {
            btn.spriteState = state;
        }
    }
}