VS 2010 / Setting Image Button URL In Code Behind?

I created an image button in my header template. When clicked, it calls the Sorting method. When I change the image url in code behind, it does not render. By setting a break point, I can see the property being updated, but I assume it is getting overridden by the default I set in the HTML page.

Code:

<HeaderTemplate>
<span>Claim Id</span>
<asp:ImageButton ID="btnClaimIdSort" runat="server" ImageUrl="~/Styles/UpArrow.ico" OnClick="Sorting" ToolTip="ClaimId" />
</HeaderTemplate>

Code:

protected void Sorting(object sender, EventArgs e)
{
var sortButton = (ImageButton)sender;
// parse new sort
var newField = sortButton.ToolTip;
// parse existing sort
var existingSortArgs = this.ClaimSort.Split(' ')

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