Here comes the solution my friend.
function ChangeImage() {
var img = document.getElementById('<%=ImageButton1.ClientID%>');
img.src = 'Images/drag.gif';
}
<asp:ImageButton ID="ImageButton1" runat="server" />
<asp:Button ID="Button1" runat="server" OnClientClick=" ChangeImage()"
Text="Button" />
Here what i am doing is that when i click on the button i called the function changeImage to change the source of the image button image.
Good Luck !