add
return false;
after
document.getElementById('<%=img.ClientID %>').src = 'spinning-wheel.gif';
and also try this when calling
OnClientClick="
javascript:Change();"
becuase it cuases full postback if we don't use "return false" statement.
function Change() {
document.getElementById('<%=img.ClientID %>').src = 'spinning-wheel.gif';
return false;
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<div>
<asp:Image AlternateText="aaa" ID="img" runat="server" ImageUrl="~/p.jpg" />
<asp:Button ID="btn" runat="server" OnClientClick="Change()" Text="Click to Change"/>
</div>