Content property of pseudo element :before accepts images too. In slick-theme.css change:
// change_x000D_
$slick-prev-character: "?" !default; _x000D_
_x000D_
// to_x000D_
$slick-prev-character: url('image-prev.png');_x000D_
_x000D_
// and _x000D_
$slick-next-character: "?" !default;_x000D_
_x000D_
// to _x000D_
$slick-next-character: url('image-next.jpg');
_x000D_
or directly change the content property on slick-prev:before and slick-next:before
.slick-prev {_x000D_
left: -25px;_x000D_
[dir="rtl"] & {_x000D_
left: auto;_x000D_
right: -25px;_x000D_
}_x000D_
&:before {_x000D_
content: url('image-prev.jpg'); // <<<<<<<<_x000D_
[dir="rtl"] & {_x000D_
content: url('image-next.jpg'); // <<<<<<<<_x000D_
}_x000D_
}_x000D_
}_x000D_
_x000D_
.slick-next {_x000D_
right: -25px;_x000D_
[dir="rtl"] & {_x000D_
left: -25px;_x000D_
right: auto;_x000D_
}_x000D_
&:before {_x000D_
content: url('image-next.jpg'); // <<<<<<<<_x000D_
[dir="rtl"] & {_x000D_
content: url('image-prev.jpg'); // <<<<<<<<_x000D_
}_x000D_
}_x000D_
}
_x000D_