How to give animation to the on gui button?

It seems you cannot animate the image inside the button as it takes a texture and not a material. What you could do is store the animation in single textures and animate them by code:

 public Texture2D[] textures;
 float timer;
 float freq;
 public float frequency;
 private int index;
 void Start()
 {
    freq = 1/frequency;
 }
 void Update()
 {
    timer += Time.deltaTime;
    if(timer > freq){
        timer = 0;
        if(++index == textures.Length)index = 0;
 
    }
 }
 
 void OnGUI()
 {
    if(GUI.Button(rect,"", textures[index])){}
 }