Why you cant use default CSS classes and add some your style?
.slick-next {
/*my style*/
background: url(my-image.png);
}
and
.slick-prev {
/*my style*/
background: url(my-image.png);
}
Are you used simple background css property?
in example: http://jsfiddle.net/BNvke/1/
You can use Font Awesome too. Don't forget about CSS pseudo elements.
And don't forget jQuery, you can replace elements, add classes, etc.