I cannot change the sprite the of the image with script

I have been trying to change the image when a button is pressed, but it always says:

NullReferenceException: Object reference not set to an instance of an object

I already added a sprite to the public Sprite variable but it is still the same.

***This object is an object I created as an empty game object and added the script to it. It also has an image as a child.***

This is my code:

Code (csharp):

1.  
2. public Sprite firstImage; // Already added sprite to this in the game
3. public...

I cannot change the sprite the of the image with script