wordpress post pagination with image buttons?

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! 🙂