You can change this button using custom CSS theme

1. Upload an image to your server.

2. Add the following CSS code to your store:

/* The "Add to bag" button in up state */
div.AddToBagButton-up, div.AddToBagButton-up-hovering {
background: transparent url(icons/add_to_bag.png) no-repeat top center !important;
}
/* The "Add to bag" button in down state */
div.AddToBagButton-down, div.AddToBagButton-down-hovering {
background: transparent url(icons/add_to_bag.png) no-repeat bottom center !important;
}   

3. Replace the icons/add_to_bag.png with the real absolute path to the modified image. For example: url(http://www.example.com/images/my_custom_add_to_bag.png)