function my_custom_follow_button_args( $button_args, $leader_id, $follower_id ) {
    $button_args['link_text'] = '<img src="LINK TO IMAGE HERE" alt="" />';

    return $button_args;
}
add_filter( 'bp_follow_get_add_follow_button', 'my_custom_follow_button_args', 10, 3 );