addClass should only apply to the next IMG

Question

I hope somebody can help me. how can I make addClass applies only to the next img and not to all following? edit: the "addClass" should always apply to the next following IMG so that you can click through all images one after an other - like a gallery

$("#next").click(function(){
    if($("#gallery li").next("#gallery li").length != 0) {
        $("#gallery li").children("img").removeClass('toppicture');
        $("#gallery li").next("#gallery li").children("img").addClass('toppicture');
    }
    else {
        $(this).children("img").removeClass('toppicture');
        $("#gallery li:first-child").children("img").addClass('toppicture');
    }
});

the ul with the id "gallery" looks like this :

<li>
    <img class="toppicture" src="images/w1.jpg" title="Title #0"/>
</li>
<li>
    <img  src="images/w2.jpg" title="Title #1"/>
</li>
<li>
    <img  src="images/w3.jpg" title="Title #2"/>
</li>

Answer 1

Have you tried :

$("#next").click(function(){
                if($("#gallery li").next("#gallery li").length != 0) {
                    var obj = $(".toppicture").parent("li").next().children("img");
                    $("#gallery li").children("img").removeClass('toppicture');
                    $(obj).addClass('toppicture');
                }
                else {
                    $(this).children("img").removeClass('toppicture');
                    $("#gallery li:first-child").children("img").addClass('toppicture');
                }
            });

source : http://api.jquery.com/first-selector/

Answer 2

Maybe you could try

$("#next").click(function(){
    //find the li with the class
    var liTop = $("#gallery li.toppicture");
    //if it has another element remove the class from itself and add it to the next 
    if(liTop.next().length !== 0) {
        liTop.removeClass('toppicture');
        liTop.next().addClass('toppicture');
    }
    else {
        $(this).children("img").removeClass('toppicture');
        $("#gallery li:first-child").children("img").addClass('toppicture');
    }
});