To change add to cart button to custom image in Squarespace, you can use this code to Custom CSS box.
/* Add to cart button custom image */
div.sqs-add-to-cart-button {
background-image: url(https://cdn.pixabay.com/photo/2023/09/26/17/32/woman-8277925_1280.jpg);
background-size: cover;
background-repeat: no-repeat;
background-position: center center;
}
Result
