here is the code. this works perfectly fine for me. do not add any code in codebehind file.
<script type="text/javascript" language="javascript">
function ChangeImage1(Image1) {
var Image2 = $get('<%=btnImage2.ClientID%>')
var tempURL = Image2.src;
Image2.src = Image1.src;
Image1.src = tempURL;
return false;
}
function ChangeImage2(Image2) {
var Image1 = $get('<%=btnImage1.ClientID%>')
var tempURL = Image2.src;
Image2.src = Image1.src;
Image1.src = tempURL;
return false;
}
</script>
<tr>
<td>
<asp:ImageButton ID="btnImage1" runat="server" CausesValidation="false" ImageUrl="../Images/btnSave.png" OnClientClick = "JavaScript: return ChangeImage1(this);"
/>
</td>
<td>
<asp:ImageButton ID="btnImage2" runat="server" CausesValidation="false" ImageUrl="../Images/btnUpdate.png" OnClientClick = "JavaScript: return ChangeImage2(this);"
/>
</td>
</tr>