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Toggle Image

hello i got the following situation

Ive got an link which opens a div onclick
This link has a class for an image.
Now i want to change this image if the div is opened or closed.
Jquery has to change the class of the link between "open" and "close"

I cant get it working

Here is the link:
<a onclick="ShowHide('#box-ext-order-id'); return false;" href="#" class="open">&nbsp;</a>

Here is the jquery script that opens the div

function ShowHide(divid){
  $(divid).animate({"height": "toggle"}, { duration: 200});
}

Anybody an idea how I can change the class of the link?
There are also some other boxes so I can not replace all classes. Only the one of the link i just clicked on

many Thanks