Hello,

Please add this code to the Theme Settings > Custom CSS > Global:

.product-label.new {
    background-color: transparent;
    color: transparent;
	background-image:url(https://woodmartcdn-cec2.kxcdn.com/wp-content/uploads/2016/07/blog-11-75x65.jpg);
	background-repeat:no-repeat;
}

Replace the image URL with yours one.

Best Regards