Why can't I call an HTTP Handler through Ajax?

I have a very simple page. An image that gets populated on the server side by an httphandler.

I don't want the ENTIRE page updated, so I wrapped that image with a button in an update panel. Then it stopped working. Why?

Here is the image that points to the httphandler:

 <asp:ImageButton ID="imgbtnImage" runat="server"  
I开发者_C百科mageUrl='<%#"~\Handlers\Handler.ashx?id=23%>' CommandArgument='23'/>