如果我理解正确，你去找endRequest事件 - 以避免图像的onload事件。有了这个code你在JavaScript端的面板更新的事件。

If I understand correct you look for the endRequest event - to avoid the onload of your image. With this code you get the event of your panel update on javascript side.

var prm = Sys.WebForms.PageRequestManager.getInstance();    
prm.add_initializeRequest(InitializeRequest);
prm.add_endRequest(EndRequest);

function InitializeRequest(sender, args) {      
}

function EndRequest(sender, args) {
}