<img src="/images/arrow_down.png" id="#1" class="nav-toggle">
<div id="1" style="display:none">some content</div>

jQuery('document').ready(function($) {
    jQuery('.nav-toggle').click(function() {
        var collapse_content_selector = $(this).attr('id');
        var toggle_switch = $(this);
        $(collapse_content_selector).toggle(function() {
            if ($(this).css('display') == 'none') {

            } else {
                toggle_switch.attr("src").replace("down", "up");
            }
        });
    });
});​

toggle_switch.attr("src", toggle_switch.attr("src").replace("down", "up"));

<a href="#" id="#arrow-toggle">
    <img src="/images/arrow_down.png">
    <img src="/images/arrow_up.png" class="up" style="display: none;">
</a>
<div id="content" style="display:none">some content</div>

(function($) {

    $('#arrow-toggle').click(function(event) {
        event.preventDefault();
        $('img:visible', this).hide().siblings().show();
        $('#content').toggle($('img:visible').is('.up'));
    });

})(jQuery);

jQuery('.nav-toggle').click(function() {
        var collapse_content_selector = $(this).attr('id');
        var toggle_switch = $(this);
        $(collapse_content_selector).toggle(function() {
            if ($(this).css('display') == 'none') {
                toggle_switch.attr("src", toggle_switch.attr("src").replace("image_2.jpg", "image_1.jpg"));
            } else {
                toggle_switch.attr("src", toggle_switch.attr("src").replace("image_1.jpg", "image_2.jpg"));
            }
        });
    });