About the wp_link_pages functions you can read in the WordPress Codex http://codex.wordpress.org/Function_Reference/wp_link_pages.
Here you go:
wp_link_pages(
array(
'before' => '<div class="pagenav">',
'after' => '</div>',
'next_or_number' => 'next',
'nextpagelink' => __('<img src="https://wordpress.stackexchange.com/questions/56190/IMAGE SRC HERE" />'),
'previouspagelink' => __('<img src="https://wordpress.stackexchange.com/questions/56190/IMAGE SRC HERE" />')
)
);
Good luck! 🙂