PHP code to use on next & previouspage links

adjust the ‘nextpagelink’ and ‘previouspagelink’ arguments for wp_link_pages() to have a css class you can style

$args = array(
    'nextpagelink'     => '<span class="next">' . __('Next page') . '</span>',
    'previouspagelink' => '<span class="prev">' . __('Previous page') . '</span>'
);
wp_link_pages($args);

then in your css you can style those spans to use your arrows as background images.