here is another example for changing the arrows and using your own arrow-images.
.slick-prev:before {
background-image: url('images/arrow-left.png');
background-size: 50px 50px;
display: inline-block;
width: 50px;
height: 50px;
content:"";
}
.slick-next:before {
background-image: url('images/arrow-right.png');
background-size: 50px 50px;
display: inline-block;
width: 50px;
height: 50px;
content:"";
}